Skype for Palm OS
http://www.palminfocenter.com/news/8973/new-solution-for-skype-on-palm-os-treos/
MSN for Palm OS
GF Messenger
http://www.3gfan.com/soft/55.htm
http://mytreo.net/archives/2005/07/interested-in-instant-messaging-on-your-treo-this-article-is-for-you.html
Discussion
http://bbs.tompda.com/thread-2102608-1-14.html