I use a Haicom Bike Holder for N900 (picture on ebay and pictures with the phone). As you can see in the pictures; the phone fits perfectly in the mount. Also it's very easy to take the phone out (even one-handed), because you can push the "left part" of the mount open with a spring. IMO that's an amazing design; when i get a call when on the bike i can just take my phone out with one hand

The mount is adjustable, so you can place it in portrait, landscape (or diagonal )
It doesn't cover the audio jack, screen lock button, usb port. Unfortunately it covers the power button, and aditionally the volume button OR the camera button (depends how you place your phone). I have it set up so that it covers the camera button, so that i can access the volume button.