I can't think of any particular advantage to mounting the N900 in portrait mode.
If you think about it, portrait mode is really only a help to onehanded operation, i.e. when you only have one hand free to both hold & operate the unit.
With a Dashboard mount, the unit is held for you so the orientation is (far) less important.

You could argue that certain applications work better in portrait, but I would suggest that there are really only 3 critical N900 apps used in-car (i.e. whilst driving):
The Phone, Sat-Nav and Media-Player.
In the limited interaction mode that (should) characterise use whilst driving, all 3 work just fine in landscape.
</2cents>

On the other hand - if Nokia release an OS-level portrait mode in the next update (or two), then I'm sure some enterprising soul can figure out a way, to invert the base co-ordinate for the Portrait-mode, so that the screen is then presented upside-down - which would put the USB port at the "bottom" - better yet, if they linked it to the accelerometer, then you could have the screen auto-invert