I am also having some problems with sleep. Sometimes it works
automatically, sometimes it doesn't and I have not been able to find
anything running that correlates with that. Right now it's a mystery.
For no particular reason, you just reminded me of a period when I couldn't
get sleep to work automatically, and when sleep wasn't set, even the screen
saver would fail to happen.
I then noticed that if I set the timeout to less than 5 minutes, screen
saver or sleep would work, and eventually I began to suspect my wireless
mouse and keyboard. I plugged in my original wired items, and the problem
went away. I had already turned off the Wake on LAN and similar settings to
no avail.
Other KB/M brands were OK.
To summarize the above long story: some devices (in my case a wireless
mouse/keyboard combo) call home periodically, which would wake Windows up
or make it think the computer was in use.