Back when I gave the BlackBerry (Part 1 and Part 2) a shot during the Smartphone Round Robin, there were two BB apps that impressed me: Opera Mobile and JiveTalk. JiveTalk is a multi-IM client and it was by far the best one I tried for the BlackBerry. Over at the CrackBerry forums there was near-universal consensus that it is the best IM client for the platform.
There are a ton of IM clients out there for Windows Mobile (including many on-deck clients built by OZ), but what's neat about JiveTalk right now is the clean, finger-friendly interface. Now remember, this thing is in Alpha and so you're going to see some bugs and it's not yet available for Standard Edition, just Pro. You'll want to keep an eye on the BeeJive Blog for the latest.
Still and all, looks promising, you might want to check it out: JiveTalk Alpha
Thanks to Merlyn3D for the tip!