Would anyone know if it's possible to write bots that talk on Google Talk (e.g. react to user input to play games etc.)?

It should be. Google talk is all XMPP, and there are libraries for that available for most languages.

If you have coldfusion mx 7 enterprise you can write one in 10 minutes. It's awesome!

I've made such bot couple of years ago (of course GTalk didn't exists than, but Jabber network did) with http://ekg2.org client (running as a demon) and python scripting.