The kickstand comes with the N900, and the manufacturers of most hardcases (like the Otterbox) make sure you can still use it while the case is attached.
Btw: for more info about the Otterbox for the N900 + some (bad-quality) pictures see also this thread.

The Otterbox (and other plastic or aluminium hardcases) make the unit larger and therefore almost impossible to fit in most N900-specific horizontal or vertical pouches/cases that you attach to your belt. If you want to use the Otterbox + a belt-pouch, I think that your only option is a generic pouch, or a pouch for an other (larger) gadget (camera, MP3-player, Nintendo DS, etc) that somehow fits the N900.

And I never use the kickstand, although it may come in handy if you want to watch movies during a long flight.