Thing is, Longhorn server has to be a REALLY strong contender. By the time it is out Linux/FreeBSD will be even better than they are at the moment, it is hard to justify buying a windows server for anything other than domains; with a double hardware cost
and much higher cost in general (even compared to non-free RedHat etc).
I have nothing against samba, but managing it can be a pain.. So at the moment it is not an ideal solution but it is just getting better and better with each release and will not be too long until it is equal (or better) than Windows server.