Bluetooth stereo is the way to go.
I have a Jabra BT 620's had them for over a year back when I had the T-Moble MDA. Work great for listening to music or ANY audio from the devices.
For the car, I overwhemingly recommend the Motorla T505. It's a BT to FM transceier. Mounts on your car's visor - your phone links to that, then it transmits that audio to stereo FM, has nice circuitry that finds the best empty FM channel for you.
Because the Omnia's connector is on the side, and the propietary connector doesn't look like the toughest thing in the world. I want to limit it's use to just charging the device personally.
Enjoy!