I have the above phone and have experienced the same bluetooth voice dialing w/handset in holster/case problem. It disconnects when voice dialing through bluetooth while in case...And I've tried it with 3 different bluetooth's from 2 different manufacterers (Jabra & Motorola).
My case has a magnetic clasp to hold it closed, and whenever I use that case I can't voice dial, but when I switched to my old velcro closure case, I can voice dial w/bluetooth just fine!
The blackberry cases have magnets on them. Once you slide the phone into the case, the phone recognizes that it is in the case and goes into power saver mode. You can do a little test by sliding the phone into the case, it will go into power saver mode, slide the phone out and the screen will light up again. The velcro case of course doesn't have a magnet in it, so the phone will not recognize wether it's in the case or not. You can always change the setting to not recognize the magnetic case. Hope this helps.