Mine is more than happy in my pocket, and I have it in a flip case that's a bit more bulky than the CP-321 Duffer mentioned - That said, I come from an E90 so by comparison the N900's tiny anyway.
To me, the N900 is much more pocketable than a lot of touchscreens on the market - It's thickness adds bulk, but as it isn't as wide as most it slips into pockets easily (shorter and less wide than an iPhone for example, just thicker).