Matthew -

I got a Plantronics convertible (I forget the model, but something like M230i) - has a USB adapter with a 2.5mm plug at the end; the headset itself has a 2.5mm jack with mic and one ear covered (I like to be able to hear what's going on around me....) 

So the headphone is usable apart from the USB adapter (like right in the headset jack on a phone) and can be replace with any headset with a 2.5mm jack (like when I rip it apart forgetting I'm tethered).  It was like $20 at Circuit City six months ago...

Works in Linux just fine (at least my Ubuntu 8.1).
Chris.