With the S60, S80, and new S70, leaving the key in the ignition is leaving the lights on. Inserting the key turns on a radio transmitter that interrogates the transponder embedded in the key. It's all part of the anti-theft system.
It is important, when you use Valet parking, to inform the attendant. Most of the Valets at hotels already know about new Volvos, and will remove the keys, but restaurants are another story. You will end up with a dead battery every time.

Dealers should know about this; Volvo issued internal bulletins beginning with the '99 S80. You can find them listed on the VCNA web site.