[schettj]

It's the WinMo insomnia effect.

Here's one reason it happens
* You have an app that installs a wakeup notification. Pretty much anything that needs to run periodically has to do this.
* Said app must "fully wake up" the device to function - getting a GPS fix is one activity that pretty much needs the device more awake then possible with the screen off.

Net effect - device wakes up (normally this happens more often then you would think, but the screen stays off and buttons are locked, so you never know it...) and starts the app, notifies it, and then the app fully wakes up the device, and HEY PRESTO screen is on. Some time later app finishes, and power times out, and it goes back to sleep.

GPS Today (standalone or the today screen plugin) will do this. Your device will be blinking like a meth addict.

As others pointed out, flipping into roaming, or dropping from EVDO to 1x *might* cause it as well, I think the OS needs to be notified when the radio detects those changes.