I am not sure I can connect the two events, but ever since I connected a Cinema display to my 2009 Mac Pro it seems like it is not entering sleep mode after the indicated time has passed. It will Sleep just fine if I use the apple menu command.
I've searched the usual news groups and other apple web sites and while I've seen a lot of speculation I've not seen a solution. I tried turning off all sharing on this computer (which was one of the more promising tips) and that didn't do the trick.
Any suggestions from those wiser than I would be great!