I'm having no end of trouble with mine. I'm not sure if the sleep function is the only culprit, but if I let it enter sleep it will lock. Seems like the part that turns the display off is what is killing it. Hibernate works fine and I wouldn't mind using hibernate, but it has to run through bios again, reconnect with internet, etc.
This started after installing an ATI-based 5770 video card. MS engineers are supposed to call me on Dec 1.