Did you know that AOL’s network for Instant Messaging (AIM), which iChat talks with, has been deemed a monopoly? Next time AOL upgrads AIM they’ll have to make it interoperable with other networks. So they’re not going to upgrade it. This appears to mean, among other things, that iChat’s not likely to get nifty features like grouped buddies so you can be “away” to workmates and “available” to friends.
If you want to message across networks, you can use a client like Fire for OS X or Trillian for Windows that lets you connect with all of them – but it won’t support audio and video yet, because each network has a different protocol.