After beating my head against a wall for the better part of an hour (I’m on vacation, I had the time to kill), I was completely unable to get iChat to connect to GTalk via the Jabber configuration that used to work prior to my Snow Leoapard install.
After searching around (on Google’s site), I resorted to trolling the Google Talk support forums and stumbled on the solution; you have to use a Captcha unlock to allow the signin to work. Why, I have no idea, but thank you to the person who posted this link on the Google forums.
If you are trying to sign into iChat and constantly get the “Login information is incorrect” message, go here: https://www.google.com/accounts/UnlockCaptcha. I have no idea why you have to do this or what else becomes unlocked through this, but it allowed me to finally log in with iChat 5 on Snow Leopard.