I've never seen anything but a couple of web-based adverts for the N900 - I've never seen a N900 advertisement in any other form of media over here in the UK. Really not sure who Nokia are aiming it at, but clearly they're not yet going for the mainstream buyer and with such a low profile I can't imagine sales have been exactly stellar (now that the initial post-launch rush has died down).

Good to know that some adverts are appearing on the Underground though, I'll keep my eye out... if you see anyone using an N900 on the Northern Line, it's probably me...