My new 08 XC90 just had a problem with a dead battery. The dealer couldn't figure out what was the cause. Seems to be fine now.

Any one out there had a similar experience?

Yes I had a similar experience with my 07 3.2 AWD. I had to disconnect battery and charge overnight - couldn't even find it at first :)

Anyway since then I've been leaving switching to parking lights, low fan, no seat heaters, etc. to preserve battery. I haven't taken it in to dealer yet, but will soon.

Try this I've identified a problem with my XC 90:
Turn on engine, switch to parking lights only. Pull into your garage (or a public parking garage after dark, so you can pull up very close to a wall in front of you. This way you should be able to see whether the lights have a mild flickering effect (your xenon lights will not flicker - just the incandescent lights). Try varying engine speed to see if this effects flicker - if they do flicker, and engine speed does not reduce the flicker, it could indicate a charging system problem - and time to take it to the dealer.

Since I only had 2500 miles on it I called the Volvo hotline and got a free tow to the dealer. The dealer had it for a week and could not dupicate it. But they did tell me that if I leave the car for more than two weeks the battery will be dead when I get back. The dam thing has 13 computers that draw power, god knows what they do!! The guy at the dealer thinks it might have been the dome light and how it was sequenced but I think he is guessing. I am wondering if Volvo has a trickle charger like Mercedes? Need to check there accessory section. It should be a standard feature that is basic with the car and not an extra.