SENA has made a vertical pouch available for the N900:

http://www.senacases.com/nokia-cases/nokia-n900-cases/

It's quite nice, and a perfect fit, but doesn't provide much access other than the headphone jack. The belt clip is somewhat vertically fixed - not completely racheting as stated on the site, but with some movement. The latch is magnetic, which I was initially weary about, however it doesn't seem to be causing any issues or problems.

I've asked SENA if they will be coming out with an UltraSlim for the N900. They're looking for some indication of demand before heading up this route. I'd prefer if companies would take notice of their own market, but maybe drop them a message if you're interested in an UltraSlim. I'm hoping to get one for those pants-pocket out-and-about days...