Alright, I've tried all kinds of different configurations with jabber and I don't think there's any way to get the same seamless functionality that AIM and YahooIM provide. I can probably write something specifically for it, but it would still need a dedicated server to run the transport.
The easiest option I've found is just to use jabtxt.com. The good thing about it is that it provides two way SMS messaging and will work on any phone regardless of OS. You don't have to run any extra software on your phone and there is no battery drain.
The downside is that you wont be using the same chat ID. Your friends will have two listings of you on their contact list. One would be the usual gChat ID, which they'd use when you're at your computer, the other would be your mobile ID which would look something like this (with sprint):
5555555555.sprintpcs@jabtxt.com
Which they can rename to anything they want. When you receive a gChat message from them on your phone, the from number will look like this:
username@gmail.com.jabtxt.com
And you can save that to your phone as a contact. That's basically what you'll put in the phone number area when you send them a text message.
Other than your friends having to deal with two contacts on their chat list, it works perfectly... this is what google should have done in the first place.
So the easiest way to get started is to send a text message to each of your friends using the phone number format listed above. Just their gmail address and then add the .jabtxt.com suffix to it. They'll get an authorization request and then it's on. Mobile gChat whenever you want it.
Enjoy.