We have mentioned Trillian working on a BlackBerry instant messaging app a few times and the app is finally available for public beta. This app has some cool features ranging from support for practically every IM standard including Jabber which is cool.
Check out the app and report back what you think in the comments. From what I have seen so far the app shows promise but this version is a very limited beta. Keep in mind the beta will expire when the app launches and will then cost about $5-10 from what I can tell.