[CentOS] Processes stuck in futex()

I'm occasionally seeing processes on my CentOS 4.3 system get hung up,
which strace reveals to be waiting on a call to futex().  Previously
I'd only ever seen this in mysql on RH9, and google searches for
explanations haven't turned up anything useful.

Any pointers to where I should look?  Is this another one of those
things where using the default SMP kernel with a hyperthreaded
processor is just a Bad Idea?