I have a 2006 Macbook running Snow Leopard. I suddenly noticed that my computer's battery would be at a different level when I woke it up than it was when I closed the lid to put it to sleep. After leaving it on a table next to my bed a few times, I realized that my computer was randomly waking up (evidenced by the Apple logo lighting up) when it was laying on a flat, level surface with nothing on top of it or anything touching it. I tried switching the settings so that I have to click the "sleep" option in the menu rather than relying on it to automatically go to sleep when the lid is closed, but the problem persists. Why won't my Macbook stay asleep?
One possible cause is that the sensor to detect lid opening can trigger by mistake. If so, you can disable it by going into Terminal.app and typing:
sudo pmset -a lidwake 0
Note that this means the Mac will no longer automatically wake when you open it. Instead you would need a key press or mouse click to wake.
What network sharing settings do you have enabled? I found that enabled internet, file or printer sharing will prevent full sleep.