I think it is already well known that your old GMail account does not function under 10.5 iChat's Jabber. The solution to this problem is to delete the old Jabber settings and to create new Google talk account within iChat.

The problem is, though, if your account does not have Admin rights. For some reason, only the Admin user can create those (Google Talk and Jabber) accounts. For security purposes, my daily account is not an Admin account. To set up these accounts, I had to give myself Admin rights (in the Accounts System Preferences panel) to fix my Google Talk problem, and then reset them when done.