I have a "high performance" machine that wakes up periodically to update media center and run a few scheduled tasks, and it always goes back to sleep within a few minutes of the last task's completions. I believe you when you say your box won't go to sleep, but I am running the same OS code as you and mine does. The only difference between our machines is likely to be drivers...
Power Scheme GUID: 8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c (High performance)
Subgroup GUID: 238c9fa8-0aad-41ed-83f4-97be242c8f20 (Sleep)
Power Setting GUID: 29f6c1db-86da-48c5-9fdb-f2b67b1f44da (Sleep after)
Minimum Possible Setting: 0x00000000
Maximum Possible Setting: 0xffffffff
Possible Settings increment: 0x00000001
Possible Settings units: Seconds
Current AC Power Setting Index: 0x00000000 // <- Sleep after: never
Current DC Power Setting Index: 0x00000000 // <- Sleep after: never
Log Name: System
Date: 8/5/2010 6:27:57 PM
Event ID: 1
Task Category: None
User: LOCAL SERVICE
The system has resumed from sleep.
Sleep Time: 2010-08-05T13:21:26.782877800Z
Wake Time: 2010-08-05T22:27:47.921200700Z
Wake Source: Timer - Windows will execute '\Microsoft\Windows\Media Center\mcupdate_scheduled' scheduled task that requested waking the computer.
...A few SCM/7036 and 7040s here for the WMC Scheduler and BITS services, and then...
Log Name: System
Date: 8/5/2010 6:32:19 PM
Event ID: 42
Task Category: (64)
The system is entering sleep.
Sleep Reason: System Idle
It's only by design if YOU wake the machine (or the computer thinks you did). If you wake it via the keyboard, mouse, or power button, technically that's a user wake and the sleep timer is reset. If an application or service did the waking, it SHOULD go back to sleep (as my box does), and not honor anything about the power plan. Only user-initiated events will keep the system awake on High Performance / Sleep after: never, so it'd be interesting if you ran powercfg -lastwake
after your machine wakes and won't go back to sleep to see what actually woke the box. Technically, if your box was woken and won't go to sleep, that should be a user event like the mouse, a keyboard, or the power button as the reason.
There is also a powercfg command (that MS should have had you run) that can determine if anything would keep your box from sleeping as well after a wake event, powercfg -requests