Comment 8 for bug 195975

I have experienced exactly the behaviour as described by doobydave "I experience the reverse of this..."@2009-04-18 on a fresh install of Oneiric. Drove me crazy for a few days.
The problem is that all the local traffic is still being routed (or at least tried to) via the wired interface. Since the nameserver is set to the IP address of the router, DNS traffic will fail, as well as other local (192.168.1.x) traffic. IP-based internet traffic (skype or pinging IP addresses) will however continue. The eth0 interface will keep its IP-address, which is not a problem, but what is a problem is that somehow ubuntu tries to route traffic through it even when it's disconnected.

dmesg knows it:
[10327.863977] tg3 0000:09:00.0: eth0: Link is down

Output of lspci -v:
09:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5755M Gigabit Ethernet PCI Express (rev 02)
 Subsystem: Dell Device 01f9
 Flags: bus master, fast devsel, latency 0, IRQ 45
 Memory at f1ef0000 (64-bit, non-prefetchable) [size=64K]
 Expansion ROM at <ignored> [disabled]
 Capabilities: [48] Power Management version 3
 Capabilities: [50] Vital Product Data
 Capabilities: [58] Vendor Specific Information: Len=78 <?>
 Capabilities: [e8] MSI: Enable+ Count=1/1 Maskable- 64bit+
 Capabilities: [d0] Express Endpoint, MSI 00
 Capabilities: [100] Advanced Error Reporting
 Capabilities: [13c] Virtual Channel
 Capabilities: [160] Device Serial Number [snip]
 Capabilities: [16c] Power Budgeting <?>
 Kernel driver in use: tg3
 Kernel modules: tg3

The kind wireless connection doesn't matter: I tried three different cards...
Laptop is a Dell D630.

Might be related to https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/277063