I use my N900 like that every time I'm in the car.

Modrana up for navigation, FM Carkit activated to turn on Fm transmitter for music (& route calls hands-free through the radio), music played through either OMP or Symphonie (Symphonie will shuffle in 1 folder).

The phone holder is mounted barely overlapping the right edge of the windshield mirror, so I can keep one eye on the road if I need to operate the N900.

There may be some N900-specific gooseneck suction cup holders on eBay; I got a new case from there recently.