What "UNIX" AIM are you guys talking about? On AOL's web site, I see a Linux Beta client. That's for i386 type machines running Linux, not for Mac OS X at all!
In general, many command line applications designed to be portable among various UNIX or UNIX-like operating systems will easily _port_ to Mac OS X. You have to have the source code to the application and recompile, possibily changing makefiles and/or source code to get those applications to run. It's not hard to do for many beginning programmers if the port is easy, but it certainly isn't for most end users.
Even if you had the source code to the Linux client, you then have another problem... that client relies on the X Window GUI system, not Mac OS X's GUI mechanisms at all.
Anyways, right now, run AOL's Mac client inside of Classic or run Eric Peyton's Fire.app. Eric even distributes source code to Fire, so if there is something you don't like about, feel free to enhance it.
..Bill Chin