This is usually a problem with programs that change the driver for the power state and when the machine tries to switch the power state it hangs.
A usual cause is Windows Media Center, if it was configured as an server or extender will do that, since during setup it removes those states to avoid the computer being offline.
However, I have not been able to fix it myself since I did not almost spend time trying to fix this, but you can try switching power schemes.
I can try when I go home, since my computer doesn't hybernate anymore (because of WMC), but here is some readying for the meantime
http://support.microsoft.com/kb/841858
I wonder if there is a way to reset the windows power management.
BTW, windows 7 sleep mode (S3) is awesome. The computer can recover as fast as it can recover from a screen saver