I was having no problems with the iMessage beta on Lion, but with Mountain Lion now officially released, I am seeing the same frequent disconnects on GTalk. AIM seems ok.
Update 9:00 EDT: Turns out that GTalk has a product outage today. Go figure. So... never mind!
I've been having these disconnects with iChat on Lion and then with Messages beta on Lion as well. I've been using FB, Gmail and AIM accounts in the app. Sometimes only one account disconnects, sometimes two and sometimes all three. And sometimes they won't even reconnect - in that case only a restart or killing of the "imagent" process in Activity Monitor solves the problem.
I guess I'll be seeing disconnects when I upgrade to ML as well . iChat in Snow Leopard was fine IIRC.
Just saw this on a news site, I thought it was Mountain Lion, too quick to blame!
Still, a bummer of a coincidence, I wonder how many Google chat users who upgraded to this new "fandangled" Messages yesterday are thinking Messages is the culprit.