At last years PDC (professional developers conference) longhorn was given out on DVD to all attendees. Also, if you are an MSDN subscriber you get access to Longhorn there as well.

There are other shadier ways of obtaining Longhorn, P2P, Pirate shops, mail order (you gotta be a nuttcase), and so on...

That said, it's really not stable enough and really any better than XP at this stage. In fact it creates a lot of issues and so on. Driver support, windows update support and software support are nonexistent. So you have to find your own fixes and so on...

But it is available.