I have read many posts on this, but have a new issue ...

Microsoft have a knowledge base article on the topic
[url=http://support.microsoft.com/kb/927821]A computer that is running Windows Vista appears to sleep and then immediately wake[/url]

The advice does NOT solve my problem.
The relevant details:

1. No devices are armed to wake my machine from sleep -- when I execute the command "POWERCFG -DEVICEQUERY wake_armed" no deivces are shown.

2. I put the machine to sleep and it wakes 'immediately' (approx 30 secs according to system logs to go to sleep and wake up)

3. I try again, but this time I unplug a generic USB hub that has no devices on it (the hub is a part of my DELL 2001FP monitor) -- this time it works and the system stays asleep!

ANy ideas on how to remedy (aside from the obvious not using the hub on the monitor!)