Q: My Mac don't go sleeping since Lion Update

So perhaps you don't have problems to sleep your computer.

Any PDF printers then like Adobe?

I have no printers set up. I only have some icloud stuff on, but I have turned it all off and it stil will not go to sleep.

Thanks Andyzzz.  Your printer suggestion was spot on.  I didn't realize a job was trying to print.  Either Apple or or Printer drivers should address this.

yes! clearing print queue fixes my problem that hard disk cannot go to sleep

If you only have the issue, when an external monitor is connected, check here: https://discussions.apple.com/thread/3194909?start=45&tstart=0

ARGGGHH.  Something stuck in Print queue was the culprit for me.  A print job had been in there for a little over a month since my wife used my MBP to print mailing labels for Christmas cards.  But I only realized that my MBP wasn't sleeping 3 days ago.  Thanks to this thread, I checked my Print Queue and there was a job stuck there!  I deleted it, and my laptop is sleeping again with the power cord attached (it was sleeping if it was unattached.)

I have macbookpro5.1 and automatic sleep was gone.

Fixed now... by uninstalling newest iStat Menus. Sleep works now.

I found out I had an emulator running and happens with either iOS and Android emulators.  Found this out by exiting all the apps one at a time and checking pmset -g assertions until there were no assetions left.  Also got rid of any old printers that I did not recognize, but for me the real culprit was coreaudiod via the emulators.

Thank you!  This worked.  I deleted 4 old printer setups from System Preferences and my computer went to sleep!  First time in like a year.