After installing 2.2.4 update on my Pre 2, I found voice dialing through my cars bluetooth connection did not work. After careful analysis, I found that the default long distance carrier selection in the network services preference page is wrong.

In order to get voice dialing to work, you need to set the default to your carrier and not to ask.

When they introduced Skype, they had added the capability to select Skype as your long distance carrier. The default is set to ask. This causes the bluetooth voice dialing not to work.