Playing iTunes through a USB headset

I have iTunes on a laptop. (Version 5.0.1 - I'll upgrade to 6 after it has been out for a few weeks.) iTunes plays through my laptop's internal speakers just fine. However, I also have a Plantronics USB headset attached to the laptop. Windows can play audio through the headset with no problem. However, iTunes insists on playing through my internal speakers even though I have the headset set as my default playback device, and I have Windows set to use only default devices. How do I set iTunes or Windows to force iTunes to play through the USB device? (I should point out that the USB headset bypasses the soundcard - it has its own digital to analog converter built in.

Thanks!