I dont have my Sprint D yet since UPS sucks but with a Touch Pro (GSM) I had until yesterday (sold eBay) I could turn off the phone completely and somehow it would still turn on to sound alarm. I never had seen that before. I assumed you could put in flight mode but this worked turned off.
btw I used an awesome alarm ap called Klaxon that will snooze when you flip the phone face down. It will play any mp3. Get it free over at xda developers.
http://forum.xda-developers.com/showthread.php?t=412669