My understanding is that there are different BT profiles. To get all of the phone's audio (stereo media & phone) to go to your BT headset, that headset needs to support both profiles. For example, I have a Rocketfish (Best Buy) stereo bluetooth headset that I use to listen to media (music & video audio). That headset also includes a tiny microphone hole and can be used as a phone BT headset. When I first paired it to the phone it was listed as a media BT headset and a phone BT device. The problem is that its microphone sucks, so I went to SETTINGS > WIRELESS & NETWORKS > BLUETOOTH SETTINGS. With the headset connected, I long-pressed the headset's name in list of connected/detected devices and selected OPTIONS (by the way this is all on the original Droid; don't know if it's different on the Droid X). In the options menu I de-selected the phone option for that headset. Then I paired up another BT headset that I use exclusively for phone calls, and only in the car, a Plantronics Voyager Pro.
There aren't many single-ear headsets out there that will work for both media & phone, but there are a few. I think Jawbone just came out with one (but I think you need to download some software from them to enable the media function).
Anyway, that's my $0.02.
