As an update to this:
I ran powercrfg -events and powercfg -requests last night to try to determine where the hang up was, and apparently something is using my audio driver, preventing it from going to sleep properly.
I can put it to sleep via the start menu with no problems, it just won't do it automatically for some reason.
My next step is going to be to either find an updated driver that hopefully won't do this, or simply uninstall the driver and let Windows use its own driver. I've searched the net, and this seems to be a common issue with the SigmaTel HDAudio drivers.