I just installed the AgileMessenger for Smartphones ("ALPHA" build... so caution) and it is working flawlessly so far.
I used it to sign into both my Yahoo and MSN messenger accounts, and it shows all my contacts, etc (if you have a lot of contacts, the option to only show contacts who are online is cool).
Since it does both MSN and Yahoo, I can see using this rather than the built-in MSN client, unless Microsoft and Yahoo get far enough along in their IM integration that the mobile MSN messenger can talk to Yahoo messenger users. If that happens, well.. we'll see which one I like better
