Usually these sorts of problems are due to either some program running which causes the system to come back from sleep almost immediately, or it can be the sign of some bad caps on the logic board. So, any recent software changes you may have made you will want to go back and investigate. You can also try applying the just released 10.7.1 update to see if that helps. There's usually some updates included in those point releases that get lumped into "General stability, performance, and security updates".
I have OSX Lion running on an early 2008 iMac. The system has been running fine for two weeks. Today I'm having trouble with "Sleep...". Every time I hit command-option-eject disk the computer wakes up every time. Also, selecting Sleep from the Apple Menu doesn't work any more.

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