Lion - Macbook randomly wakes up from sleep when lid is closed
It is possible to determine what caused the system to wake


Open Console (/Applications/Utilities/Console)
Set the search filter to 'wake'

You're looking for the latest entry that has something like this:

Wake reason = EC LID0


(In the example above, opening the display clamshell caused the machine to be woken)


Here's a list of wake 'reasons' from OSXDaily

This should shed some light on what is waking the machine, then you can go from there.