You need to add a route manually to cover all the corporate networks. Find the smallest net block that will cover all the corporate IP addresses. For example, the corporate net might be entirely within the range 172.16.22.XXX—172.16.25.XXX. Thus your net block would be 172.16.16.0/20:

/sbin/route add -net 172.16.16.0/20 -interface en0
Note that on my machine, en0 is the Ethernet, en1 is the AirPort.

I have a script which sets up a bunch of routes for my local situation which is a corporate network for all corporatey stuff (time sheets, leave applications, workplace health & safety reporting, etc), with WiFi for the useful stuff (Bing, O'Reilly Safari, etc). I still haven't figured out where I need to put this script to have Mac OS X automatically add the arcane routes I need when the interface comes up though.