Try Agile Messenger, that's what I'm using on my WM5 device (rx1955) and I love it. It's a lot like trillian...tabbed windows, and you can be on AIM, YIM, MSNM, ICQ and a few others all in one window.
www.agilemobile.com is the site I believe... It's free, but the site says its a trial or something, but I can't find any evidence of that anywhere. No where during install does it mention trial, evaluation, or anything of the sort. Neither does the program anywhere. And it doesn't mention anything about upgrading....I have not had the program for 15 days so I'm not sure, but I think that since it's a beta, it may be free...
The only downside to the program is if you get a message while in PIE, it asks you to activate or ignore or something, and if you hit activate, it sometimes closes PIE so that you loose the webpage you were just on....an annoyance, but a small price to pay for such an awesome program IMO.