Not sure if it's something that was enabled in a software update or not, but my iPhone 3GS running the latest software update allows voice dialing from the Motorola S9-HD bluetooth headset--you hit the "voice dial" button on the S9-HD and you get the voice command prompt and use it as you normally would. (That may be the reason it works, actually--the S9 has a button for triggering voice dialing, and the iPhone does seem to know whether you are initiating the voice command mode from the headset vs. from the phone itself. So maybe other headsets without support for that specific feature built-in can't trigger the voice dialing+accept input from Bluetooth mode.)