Actually, there's a slight problem with the old fork() bomb: most OSes only allow so many running processes and there are a few that go so far as to limit the number of running processes an individual user can have, so you could, possibly, run out of processes before you run out of memory.
~Cybercosis
Update: Actually, it is somewhat required for an OS to limit the number of possible processes, since you can only hold so many TSS templates to be moved into the TSS register in memory. Well, at least with i86 processors. =-)