I have a late 2008 MBP (2.53 ghz, 250GB HDD, 4GB ram) that's been having this same problem. I'll stop using it at night and close the lid. When I open the lid in the morning, sometimes the screen stays black, though the sleep light is not fading in and out, indicating that the computer is not actually sleeping.
Multiple opens and closes of the lid, tapping space or trackpad, brief touches of power button, inserting a disc, none of these things will wake it and I have to power it down with the power button (losing my work state on anything I had open).