I can't really help you here but I'm guessing that there needs to be an update for Pidgin. I'm using Kopete and have the very same situation in gtalk. Interestingly enough, Google's own IM client for Windows doesn't even have a provision for invisible mode.
Don't remember if any option was there for becoming invisible in google talk for windows, but you get this option if you use chat feature inside gmail. Using pidgin 2.4.1 now, but updating to see if latest version solves my problem.
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