Blackcomb is the codename for the new MS server platform that should replace windows 2000 server as the standard. This product does not have a (tentative) release date yet.
Longhorn is the new MS desktop platform and is due for release in either 2004 or 2005, according to their representatives.
The March edition of CPU had an excellent article that compared Linux, Windows, and OSX as well as provided some insight about what to expect from these operating systems in the future. I highly recommend checking it out if you're at all interested.