There have been some questions regarding Google Talk and its Invisible status. XMPP/Jabber didn't support Invisible when Google Talk, which is based on XMPP/Jabber, added support for it. This was likely to create a uniform interface to the user who now had the ability to go invisible on their AIM account.
Well, there's good news. I created a plugin to enable Invisible for Google Talk accounts.
Now, before you go rushing off to download and install that plugin, here's the big caveat: I only made it work for one account. If you have more than one Jabber/XMPP/GTalk account, then it won't work for all of them. Proceed with caution, because having multiple accounts may break it. I have tested it with one account, so no worries there Fixed, now supports multiple accounts (but no guarantees, this is open source software).
Update 12/1/2011: I've updated the code with a patch from Khushman. It fixes the startup issue that would crash some people's pidgins on startup, and also includes a 64-bit version of the compiled plugin. Hope it works!
Update 6/27/2010: Again, there have been many further comments as to whether it works or not. Seeing this much demand for only a few lines of code is very interesting. Until now, many of your problems have been either unfixable due to the way the plugin works (a hack) or too much work to reproduce. However, I've been notified of a better way to go about going invisible and I will try to update the plugin soon (no guarantees however) with that new system: http://chinmaykamat.wordpress.com/2010/01/22/google-talk-invisible-mode-in-pidgin/
Sorry to everyone who's Pidgin crashes when using this plugin, there's little I can do when I have the plugin enabled all the time and get no crashes. In fact, I've had Pidgin crash repeatedly on me before, but never since I enabled this plugin.
Update 3/19/2010: There have been a lot of requests for fixes/updates/rewrites to this code. I'm sorry to say, but I've moved on to newer projects. This was originally only a weekend project for me, little did I realize that there was this much demand for it. I've provided all the code I have for this, if anyone wishes I can provide the whole working directory as well as instructions to compile on both Linux and Windows, but beyond that I will not be making any more changes.
Now, I have a patch waiting in my email, and I will attempt to incorporate that sometime in the future, but as above, no guarantees.
Sorry to everyone who has posted with problems with this code, but it's over a year old, meant for a much older version of Pidgin, and works with at least historically badly documented APIs (sorry Pidgin, but that documentation wasn't easy to understand at all, I worked mostly off examples). Good luck with getting a real invisible implementation into Pidgin trunk!