I've seen an app that can launch voice search from the BT headset, but it still uses the phone's mic. It would be great if the voice dialer could be completely replaced with the much more functional new Voice Search. If you agree, please vote over at google for this feature:
Issue 8973 - android - Add voice search over bluetooth, similar to voice dialer in 2.2 - Project Hosting on Google Code
Issue 8973 - android - Add voice search over bluetooth, similar to voice dialer in 2.2 - Project Hosting on Google Code