The tight integration with several IM services (Skype, GTalk, MSN Messenger, etc;) is one of the great features of the Nokia N900. It is super convenient. I sang about it in my first impressions review of the Nokia N900. You just enter your IM login and all your contacts of these appear in the address book. It may take some work from the merger of these contacts, especially for incomplete profiles, but they are there, and when you talk to someone, just select the mode – Skype, GTalk, SMS, e-mail or phone call.
It just works, and I changed the patterns of communication on progress significantly. These days I am doing much more IM / Skype on the road than ever before. Or was. Because I just had a major flaw in this combined approach on the Nokia N900 phone book. He lost most of my contact list of Skype and Gtalk last 6 years in the process.