The "bikeconsole" arrived today. The original product is Tigra BikeConsole Universal 4.8".
I've ordered mine from Cyclewiz located in the UK. For the US Bike2Power offers a reasonable deal.

The N900 fits nicely, there is room for a second battery or other little extras. You can access the volume keys while riding which is good for zooming the map in "marble".

As you can see in the picture, the case is freely rotatable, landscape mode is no problem. The rain protection worked too
The black sticking-out-thing in the picture will soon be cut off once I've determined the best position.

Bonus points for the integrated headphone and USB cable, I'll probably add some on-the-go charging device later on.