Earlier this month, AOL released their first custom-made IM application for Windows Mobile phones (versions 5 and 6). The application, AIM for Windows Mobile, lets you IM your friends, update your AIM status, manage your buddy list, insert emoticons into your updates, and more. In a variation on tabbed conversations, this AIM client shows the buddy icons for the people you're in conversation with on the left hand side of the screen instead. Those icons are overlaid with the user's status (busy, mobile, away, etc.) as well the number of unread messages from them. Another interesting, but yet to launch, feature is the ability to send out updates via SMS instead of IM - great for those with unlimited text messages but tight data plans as well as for communicating with friends who aren't signed in.
This app does not currently have an auto-updating feature, so you'll need to keep checking beta.aol.com to find out when new versions become available.