I just started having this issue on my 27" iMac (late 2009) running Lion with the latest update installed. I installed Lion the day it came out and never had any problems until a few days ago. Now it's happening all the time. The computer will wake from sleep but then immediately locks up. The screen will come back on, but then there is no mouse cursor and no keyboard keys will work. The computer is still running, but there is nothing you can do to except hold down the power button and reboot.
I'm thinking the only thing I have installed recently is an update to Logitech's mouse software, but since no one else mentioned this, I doubt that's the cause....
By the way, my computer is set to sleep the display after 10 minutes and the computer itself doesn't sleep for 1 hour. And since this problem is happening many times less than an hour after I leave and return to my computer, I'm not sure if it's an actual "sleep" problem or not? Just the screen is sleeping, so it sure seems like a video card/driver issue.... but who knows?