In Precise I brought up eth0 with ifup and wlan0 with NetworkManager, as shown below.  The result is two default routes which isn't a problem in itself.  The problem is that NM assigns a lower metric (0) to the wlan0 default route than ifup assigned to the eth0 default route (100), which causes the kernel to send most traffic over wlan0.  If I manually assign a metric of 200 to the "default via 192.168.1.254 dev wlan0  proto static" route then traffic is directed to eth0.

I don't know how this should be solved.

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# # network-manager is stopped at this point
# grep -v '^$' /etc/NetworkManager/NetworkManager.conf 
[main]
plugins=ifupdown,keyfile
dns=dnsmasq
no-auto-default=F0:DE:F1:6B:22:BF,
[ifupdown]
managed=false
# grep -v '^$' /etc/network/interfaces
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
	address 192.168.1.30
	netmask 255.255.255.0
	gateway 192.168.1.254
# ifconfig
lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:6853 errors:0 dropped:0 overruns:0 frame:0
          TX packets:6853 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:536455 (536.4 KB)  TX bytes:536455 (536.4 KB)

# ifup eth0
# ifconfig
eth0      Link encap:Ethernet  HWaddr f0:de:f1:6b:22:bf  
          inet addr:192.168.1.30  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::f2de:f1ff:fe6b:22bf/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:4425 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4756 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:2563417 (2.5 MB)  TX bytes:688741 (688.7 KB)
          Interrupt:20 Memory:f2500000-f2520000 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:7089 errors:0 dropped:0 overruns:0 frame:0
          TX packets:7089 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:551439 (551.4 KB)  TX bytes:551439 (551.4 KB)

# ip route show
default via 192.168.1.254 dev eth0  metric 100 
169.254.0.0/16 dev eth0  scope link  metric 1000 
192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.30 
# start network-manager
network-manager start/running, process 7004
# # Enable networking in network-manager
# NM connects to wireless network
# ifconfig
eth0      Link encap:Ethernet  HWaddr f0:de:f1:6b:22:bf  
          inet addr:192.168.1.30  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::f2de:f1ff:fe6b:22bf/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:4497 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4848 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:2580263 (2.5 MB)  TX bytes:702714 (702.7 KB)
          Interrupt:20 Memory:f2500000-f2520000 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:7878 errors:0 dropped:0 overruns:0 frame:0
          TX packets:7878 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:609728 (609.7 KB)  TX bytes:609728 (609.7 KB)

wlan0     Link encap:Ethernet  HWaddr a0:88:b4:71:83:10  
          inet addr:192.168.1.21  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::a288:b4ff:fe71:8310/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:36848 errors:0 dropped:0 overruns:0 frame:0
          TX packets:26844 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:34907153 (34.9 MB)  TX bytes:5777846 (5.7 MB)

# ip route show
default via 192.168.1.254 dev wlan0  proto static 
default via 192.168.1.254 dev eth0  metric 100 
169.254.0.0/16 dev eth0  scope link  metric 1000 
192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.30 
192.168.1.0/24 dev wlan0  proto kernel  scope link  src 192.168.1.21  metric 2
# netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         192.168.1.254   0.0.0.0         UG        0 0          0 wlan0
0.0.0.0         192.168.1.254   0.0.0.0         UG        0 0          0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U         0 0          0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0          0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0          0 wlan0
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Here's how it looks when NM controls both eth0 and wlan0:
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# ip route show
default via 192.168.1.254 dev eth0  proto static 
169.254.0.0/16 dev eth0  scope link  metric 1000 
192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.20  metric 1 
192.168.1.0/24 dev wlan0  proto kernel  scope link  src 192.168.1.21  metric 2 
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