In several places that I’ve tried (most notably at work), Pidgin would fail connecting to GTalk, probably because it goes through a proxy server. However, I found a fix online today via manast.com. Basically, go to the advanced tab and set:

• Force old (port 5223) SSL
• Allow plaintext auth unchecked
• Connect port: 443
• Connect server: talk.google.com
• Use global proxy