ubuntu automatically prefers wired connections over wireless.
I have a similar set up to you and I'm desperately trying to find a way to set my wireless connection as a high priority. My wired connection also goes to a switch and then a NAS so no internet connection down that branch.
The wired branch of my network all have 192.168.2.x address and the wireless 192.168.1.x. So basically I either need to set the wireless as higher priority or use iptables (aaargh) to stop internet packets going down the 192.168.2.x branch.
Can anyone help us out? I currently have to disable eth0 to get onto the internet...