I use Otterbox Commuter Series case on my N900 which is really nice and rugged but gives some extra bulk to already thick device. So it's somehow not pocket friendly enough. Thanks to previous poster on this thread pointing me to Nite Ize products. I found two nice vertical rugged pouches on their website, compatible with N900 in Otterbox case. They are available in nylon or oilskin(medium). Nylon is available with both velcro and magnetic closures, while oilskin only velcro. The fit is somehow tight but very secure. Build quality is nice. There is also Clip Case Sideways(horizontal) available which was reported compatible earlier(I can't confirm it's compatibility with Otterbox case installed).
Vertical nylon case available at amazon.com for $12.99.
Here is the picture of the N900 in Otterbox case inside the Nite Ize pouch: