I used voice dialling on my previous phone - nokia E65 and use it on my E71 - when driving, with a bluetooth headset. It "just works" for me. My wife also uses it with her E65.

Just press the button on the headset, the phone greets you with a twiddle sound, speak the name, it dials. press button to hang up.

It's a lot better than phones of a few years ago where you have to teach them the sound for each contact, it's now speaker independent. Quite impressive, actually. When I switched from the original and great 6310i to the E65, I spent ages trying to find out how to teach it before I realised it didn't need it!