If you're running Snow Leopard, this can be the result of Time Machine. It now has the ability to wake your system just long enough to do backups. If Time Machine is turned off, or it's happening more than a few seconds every hour, then you should probably start by running Apple Hardware Test. It's usually on the Applications DVD that came with the system, and instructions for starting it are on the disc itself. Just be sure to tick off the extended test box. Then come back in about an hour.
For some reason recently my macbook sometimes decides not to stay asleep. When I put it to sleep it keeps waking up then going back to sleep again & again. I have tried closing every application & tried restarting my macbook, but it still does it. Then suddenly it stops disobeying & sleeps fine, only to start reawakening again a few days later. I suspect it may have to do with some internet connection I am not aware of but I can't figure it out. Anybody else have this experience?