For quite a time after upgrade my OS to the latest developers preview GM Mac OS X Lion, I notice whenever I’m trying to put my macbook to sleep. Not always I’m successful to do so.
Most of the time, the screen just dimmed down but it is still running in the background. So after some investigation, I found that this problem was caused by the “Internet Sharing” function has been turned on!
So just go to Preference > Sharing and untick the Internet Sharing. You should able make your laptop back to sleep again.