I have Tom Tom on both my PDA (iPAQ) and my mobile phone (Nokia 6680) and it works really well :-) I have a little mount which fits into the ventilation grille in the middle of the dash and whichever device I'm using sits in it, alternately I have the device in my breast pocket and just listen to the voice commands.
You need the software (obviously) and some form of GPS receiver, there are a lot of
bluetooth ones around (I personally use a BT-77). PM me if you need more advice or help with the software.
p.s. If you have one of the latest Symbian OS phones (version 3) like my new one, the Nokia N80, Tom Tom etc. etc. have not yet upgraded the software to work on them, expected soon though.
BD