Cancel my "Worked for me" comment regarding this thread.
Today, June 3, 2010, the iMac again refused to sleep from the Energy Saver controls. The solution that Natthew mentioned worked for one day (or apparently worked) as a result of disabling the Guest account.
Now, we're back to square one. No screen sleep and no incoming network wakeup.
Lion has turned out be be the most quirky system I've used since 1993. I should have stayed with Tiger.
Is there a 3rd party app that will do what the system says it will do, put the Mac screen to sleep after a selected number of minutes and allow it to be woken up by the usual methods?
Thanks
--gg