Well Mountain Lions new Messages (iMessage) incorporates other legacy chat services like msn, aim etc. so I don't see why Messages for iOS couldn't do the same in the future.
If you have a Trillian Astra account Trillian is great. You can also create one for free. CVheck it out: http://www.trillian.im/ . They also have mac and windows apps and even a web client.
I use this quite often on my iPhone and I agree it is one of the better clients. I used to have it on my iPad, but an IM client on a large screen like an iPad just seemed odd to me.