Do all players spawn on the same block in SMP?
Solution 1:
In multiplayer, players will spawn, by default, anywhere within a 20x20 area centered at the spawn point. An individual player can change their spawn point by merely lying in a bed -- they do not have to go to sleep (which can only happen if all players sleep at the same time).
So the default spawn point is kind of the same. There is a large amount of info on multiplayer spawning.
Solution 2:
No.
Sorry, but in SMP the server picks a random block from the spawn chunk to spawn people. They don't spawn on the same block every time.