[IGGUS]

Not sure if it's better on this particular phone, but with other WM/HTC devices it turns on because of


Bluetooth
Email Sync
Opera Sync
Weather Sync
(Really any automated sync)
Phone Call duh
Low Battery (This I love.... battery is low so let's light up the screen and play a sound, then stay on in my pocket - a proven way to extend the battery!)

There's some others I can't think of now, feel free to add to the list.