It's pretty easy, had to test a few different servers/transports to see which was the most reliable for me (my 2g gprs is a bit unreliable) I noticed some odd behaviour on certain servers such as the N900 showing me as online even though others do not see my logged in and I would not be able to send IMs. I guess the server would timeout but the N900 would not get updated...

http://www.bigblueball.com/forums/ge...msn-yahoo.html
Follow these instructions here once you create an account you need to select service discovery in PSI and it will list all the transports (msn, aim, yahoo) etc. just enter your password and login info for the service and it will populate your contact list. After that you just need to login with ur jabber ID on ur N900. One thing to note, it seems for msn anyway that some servers will pull every single contact you ever had and ask u to authorize each. It is a bit annoying, oddly enough after testing a few servers I found some that populate your list nicely just as you have it on ur desktop with groups and all...

A listing of all the servers is located here....
http://www.jabberes.org/servers/