Through my research for a connected device, I noticed that you can get versions/ports of MSN messenger for symbian devices, and I think there's a beta out there for Treo.
you can also try QuickIM (http://www.quickim.com). QuickIM offers instant messaging on the MSN network with some unique features such as handling multiple users chat sessions (other users should be invited by the PC user in the current version). QuickIM is an one time fee software, there is no annual fee.