I'm not sure where that tactic stands legally, personally i think it causes trouble, as say you have the .co.uk version of a larger companies .com. You will get lots of misdirected emails, which if all of those got a responder back saying "yeah yeah the company who has the .com is an idiot as they didn't register the .co.uk, I did. Now it's for sale for the highest price" then your asking for trouble.
Be careful kids!! :grin: