They can't for sure. But if they detect a non-mobile user-agent or torrents they will probably accuse you of using yout N900 as a Modem.

I've been using my N900 as a modem in Vodafone Spain for more than 2 month without a problem. Theoritcally I can't do that, but the only complaint i recieved so far was about torrent usage and I stopped that.

Also there's a similar thread about this somewhere...