Google Talk not working over Corporate Wifi. What can I do?
The problem is, that my android device automatically turns off the 3g connection as soon as wifi is available.


Why don't you shut off Wi-Fi?

Since you can't control port forwarding or anything at work, there's not much else you can do.  Although, you could just ask and see whether they'd consider opening the Google Talk ports (443/5222/5223).