I tried my palm as a remote, I didn't really like it.
My setup was like this:
Palm Tungsten T with bluetooh would connect to the pc via a usb bluetooth dongle.
I couldn't find any good aps to control the pc from the palm, so I used a web interface program for Winamp. WAWI was the winamp plugin I used.
On the palm i had a web browser that would connect to the winamp page. I could load playlists, browse directories and adjust the volume somewhat.
I thought it was great, but it turned out to be unusable when driving. The Screen is way to small, sometimes the connection to bluetooth would timeout, sometimes the web server was unresponsive. All in all it sucked while driving, I thought I could touch the screen while driving but it is way to hard to hold it, and then tap the correct area on a 3x4 screen, trust me.
So now I got rid of the palm, still stuck with the bluetooth dongle (wasted $50).
With the pocket PC you do have more choices for software, i came across some that looked real good, and I did wish I had a pocket pc when I was trying this out.
I now use my Pioneer Head Unit IR remote to control the PC, works much nicer, I wish I hadn't wasted my time or money with the palm control idea. I'd say take the Pocket PC with you in the car and try to listen to and control 30Gigs of mp3 music on your headphones, you'll see how hard it is to do while driving!
Just My 2cents, but go another route for a real car pc. I'm saving cash for a xenarc.