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.