I have been trying to get this working for some time.  I can offer some observations but no answers.

1, This is the same symptom with USB and Bluetooth and other devices. 
2, On first insertion of USB cable network comes up OK. Looking at /var/log/syslog

Jul 25 13:23:43 bwing kernel: [326431.367963] usb 1-1: new full speed USB device using ohci_hcd and address 86
Jul 25 13:23:43 bwing kernel: [326431.465212] usb 1-1: configuration #1 chosen from 1 choice
Jul 25 13:23:43 bwing kernel: [326431.477055] usb0: register 'cdc_subset' at usb-0000:00:02.0-1, Linux Device, 36:ea:bb:7e:ac:8c
Jul 25 13:23:43 bwing NetworkManager: <debug info>^I[1185333823.529149] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial').
Jul 25 13:23:43 bwing NetworkManager: <debug info>^I[1185333823.635551] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_ffffffff_ffffffff_noserial').
Jul 25 13:23:43 bwing NetworkManager: <debug info>^I[1185333823.683519] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/net_36_ea_bb_7e_ac_8c').
Jul 25 13:23:43 bwing NetworkManager: <debug info>^I[1185333823.735785] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial_usbraw').
Jul 25 13:23:45 bwing avahi-daemon[4925]: Registering new address record for fe80::34ea:bbff:fe7e:ac8c on usb0.*.
Jul 25 13:23:47 bwing avahi-daemon[4925]: Joining mDNS multicast group on interface usb0.IPv4 with address 192.168.211.2.
Jul 25 13:23:47 bwing avahi-daemon[4925]: New relevant interface usb0.IPv4 for mDNS.
Jul 25 13:23:47 bwing avahi-daemon[4925]: Registering new address record for 192.168.211.2 on usb0.IPv4.
Jul 25 13:23:53 bwing kernel: [326436.318568] usb0: no IPv6 routers present

3, Removal USB cable leaves an entry for usb0=usb0  in /var/run/network/ifstate dhclient is still running
    1097 ?        S<s    0:00 dhclient3 -pf /var/run/dhclient.usb0.pid -lf /var/lib/dhcp3/dhclient.usb0.leases usb0
4, Insertion of USB Cable /var/log/syslog seems avahi-daemon misses out 

Jul 25 13:28:32 bwing kernel: [326564.547139] usb 1-1: new full speed USB device using ohci_hcd and address 87
Jul 25 13:28:33 bwing kernel: [326564.651096] usb 1-1: configuration #1 chosen from 1 choice
Jul 25 13:28:33 bwing NetworkManager: <debug info>^I[1185334113.086586] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial').
Jul 25 13:28:33 bwing kernel: [326564.662938] usb0: register 'cdc_subset' at usb-0000:00:02.0-1, Linux Device, 36:ea:bb:7e:ac:8c
Jul 25 13:28:33 bwing NetworkManager: <debug info>^I[1185334113.210916] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_ffffffff_ffffffff_noserial').
Jul 25 13:28:33 bwing NetworkManager: <debug info>^I[1185334113.269558] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/net_36_ea_bb_7e_ac_8c').
Jul 25 13:28:33 bwing NetworkManager: <debug info>^I[1185334113.286177] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial_usbraw').
Jul 25 13:28:34 bwing avahi-daemon[4925]: Registering new address record for fe80::34ea:bbff:fe7e:ac8c on usb0.*.
Jul 25 13:28:43 bwing kernel: [326569.350530] usb0: no IPv6 routers present

5, Remove USB cable and remove usb0=usb0 from /var/run/network/ifstate, reinsert USB cable and interface comes up

Jul 25 13:36:34 bwing kernel: [326784.925296] usb 1-1: new full speed USB device using ohci_hcd and address 88
Jul 25 13:36:35 bwing kernel: [326785.029234] usb 1-1: configuration #1 chosen from 1 choice
Jul 25 13:36:35 bwing NetworkManager: <debug info>^I[1185334595.131111] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial').
Jul 25 13:36:35 bwing kernel: [326785.041068] usb0: register 'cdc_subset' at usb-0000:00:02.0-1, Linux Device, 36:ea:bb:7e:ac:8c
Jul 25 13:36:35 bwing NetworkManager: <debug info>^I[1185334595.237684] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_ffffffff_ffffffff_noserial').
Jul 25 13:36:35 bwing NetworkManager: <debug info>^I[1185334595.296186] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial_usbraw').
Jul 25 13:36:35 bwing NetworkManager: <debug info>^I[1185334595.321154] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/net_36_ea_bb_7e_ac_8c').
Jul 25 13:36:36 bwing avahi-daemon[4925]: Registering new address record for fe80::34ea:bbff:fe7e:ac8c on usb0.*.
Jul 25 13:36:40 bwing avahi-daemon[4925]: Joining mDNS multicast group on interface usb0.IPv4 with address 192.168.211.2.
Jul 25 13:36:40 bwing avahi-daemon[4925]: New relevant interface usb0.IPv4 for mDNS.
Jul 25 13:36:40 bwing avahi-daemon[4925]: Registering new address record for 192.168.211.2 on usb0.IPv4.
Jul 25 13:36:45 bwing kernel: [326789.737778] usb0: no IPv6 routers present

6, Set USB interface to static, network restart, remove cable, reinsert cable, entry usb0=usb0 still in /var/run/network/ifstate,

Jul 25 13:40:46 bwing kernel: [326899.141939] usb 1-1: new full speed USB device using ohci_hcd and address 89
Jul 25 13:40:46 bwing kernel: [326899.244029] usb 1-1: configuration #1 chosen from 1 choice
Jul 25 13:40:46 bwing NetworkManager: <debug info>^I[1185334846.505753] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial').
Jul 25 13:40:46 bwing kernel: [326899.255893] usb0: register 'cdc_subset' at usb-0000:00:02.0-1, Linux Device, 36:ea:bb:7e:ac:8c
Jul 25 13:40:46 bwing NetworkManager: <debug info>^I[1185334846.598956] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_ffffffff_ffffffff_noserial').
Jul 25 13:40:46 bwing NetworkManager: <debug info>^I[1185334846.665634] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial_usbraw').
Jul 25 13:40:46 bwing NetworkManager: <debug info>^I[1185334846.667092] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/net_36_ea_bb_7e_ac_8c').
Jul 25 13:40:48 bwing avahi-daemon[4925]: Registering new address record for fe80::34ea:bbff:fe7e:ac8c on usb0.*.
Jul 25 13:40:57 bwing kernel: [326904.249517] usb0: no IPv6 routers present

ifconfig shows
usb0      Link encap:Ethernet  HWaddr 36:EA:BB:7E:AC:8C
          inet6 addr: fe80::34ea:bbff:fe7e:ac8c/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:6 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 b)  TX bytes:468 (468.0 b)

7, Remove USB cable and remove usb0=usb0 entry in /var/run/network/ifstate, reinsert USB cable and interface comes up

Jul 25 13:44:44 bwing kernel: [327007.379280] usb 1-1: new full speed USB device using ohci_hcd and address 90
Jul 25 13:44:44 bwing NetworkManager: <debug info>^I[1185335084.710528] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial').
Jul 25 13:44:44 bwing kernel: [327007.479541] usb 1-1: configuration #1 chosen from 1 choice
Jul 25 13:44:44 bwing kernel: [327007.491386] usb0: register 'cdc_subset' at usb-0000:00:02.0-1, Linux Device, 36:ea:bb:7e:ac:8c
Jul 25 13:44:44 bwing NetworkManager: <debug info>^I[1185335084.815553] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_ffffffff_ffffffff_noserial').
Jul 25 13:44:44 bwing NetworkManager: <debug info>^I[1185335084.879090] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/net_36_ea_bb_7e_ac_8c').
Jul 25 13:44:44 bwing NetworkManager: <debug info>^I[1185335084.895657] nm_hal_device_added (): New device added (hal udi is '/org/freedesktop/Hal/devices/usb_device_49f_505a_noserial_usbraw').
Jul 25 13:44:44 bwing avahi-daemon[4925]: Joining mDNS multicast group on interface usb0.IPv4 with address 192.188.211.2.
Jul 25 13:44:44 bwing avahi-daemon[4925]: New relevant interface usb0.IPv4 for mDNS.
Jul 25 13:44:44 bwing avahi-daemon[4925]: Registering new address record for 192.188.211.2 on usb0.IPv4.
Jul 25 13:44:46 bwing avahi-daemon[4925]: Registering new address record for fe80::34ea:bbff:fe7e:ac8c on usb0.*.
Jul 25 13:44:55 bwing kernel: [327012.206993] usb0: no IPv6 routers present

Summary:
1, This is not a DHCP problem, However dhcpc seems to have the -r option turned on hard. (See previous post) This should not be the case.
2, This is not only USB network layer but involves other interfaces as well.
3, This is a serious bug. It means that you cannot remove and reinsert USB and other network devices and expect them to work. It used to in hotplug :(
4, Lingering  usb0=usb0 entry in /var/run/network/ifstate is a concern
5, if you do an ifdown usb0 (Even if it is not connected) this removes usb0=usb0 entry in /var/run/network/ifstate and networking comes up next time you plug in USB
6, If you manually remove usb0=usb0 entry in /var/run/network/ifstate and networking comes up next time you plug in USB

Cheers