Downloaded google talk in the office today but the proxy server blocks the port. Back at home, I only have my powerbook with me and for now google talk only works on Windows machines. However, its suppose to be compatible with Apple's iChat that comes in all modern OS X. That's because it uses the open-sourced Jabber protocol for communications. But its bad luck as I find out that only iChat v3.0 that comes with OSX Tiger works with Jabber. All older pre v3 iChats only work with AIM's protocol.
If you're on the Mac, you could use a third party IM client such as Proteus though. I already have proteus running with MSN and Yahoo accounts, and I'll check later today when I'm home to see if it works with my google account on jabber. The server should be set to talk.google.com and on port 5223. Strange, earlier on the internet there was a site that mentions that port 5222 works, but on google's site its 5223. Maybe they have more than 1 port opened for Talk.