my plantronics headset wouldn't pair with the 3G after being fine with the original, (not exactly sure what model mine is) but I eventually got these instructions off the web:
http://www.iphonefreak.com/2008/02/plantronics-voy.html
turn off bluetooth on the iphone
turn off the headset
turn on bluetooth on the iphone
hold down the power button on the headset until the light flashes blue-red continuously.
the headset will pair on the phone and then challenge you for a code. Use 0000 (four zeros)
and it works! Amazing. (I was blaming Apple...) Now if only the GPS could be solved, I'd be happy. I haven't seen a single blue dot to date!
Anyway I hope this helps someone out there. J