I was having similar problems (USB Ethernet not working) on my Kensington dock w/video. Recently I changed situations and needed to disable the DisplayLink driver entirely (I no longer had the opportunity to have dual displays on the Mac), so I uninstalled the driver and of course the USB Ethernet worked.... on 10.6.1.
Just like "wuffe" I noticed that my USB Ethernet stopped working after the upgrade to 10.6.2, even without the DisplayLink drivers. Upon reading this page:
http://www.sustworks.com/site/news_usb_ethernet.html
... it appears that the fact that our USB Ethernet devices *ever* worked is more of a fortuitous event than anything else. (The AppleUSBEthernet driver is intended just for the Apple device, no others.)
On the bright side, the driver linked there is open source and at least for the moment is working on my Kensington dock. So I have good old eth2 back!
Anyways, this might help you and anyone else in this boat.
Just to clarify: I have NOT tried the above USB Ethernet driver with the DisplayLink driver (as I said, I'm not using DisplayLink right now). I'm just saying this might help those of you in the no-USB-Ethernet camp.