After my recent upgrade to Pidgin 2.5.5 (on Ubuntu Jaunty), GTalk mysteriously stopped working. Check out the FAQ entry on the Pidgin developer website for an explanation. The workaround, not listed there, is to change your "Connect Server" to "talk.google.com". Pidgin will then prompt you once for a certificate, and after that, it will connect fine.
A developer in #pidgin on irc.freenode.org told me to "fix my router" since my "router was broken". This even though the problem has now occurred on three separate LANs, two of which I don't own/control. Routers that are used as DNS servers are very common, and the fact they are broken in this regard is a reality. Wake up — realities trump ideal every time. Pidgin should automatically work around this problem, IMO.