This is a very big grey area, to the best of my knowledge it is not illegal to register a mis-spelt domain as such BUT (and this is a big BUT)...
If the owner of the properly spelt domain can PROVE that you registered the domain simply to profit out of the mis-spelling then the chances are the domain name registry that is authoritive for that domain can (and will) strip the domain from you. This especially applies to .co.uk domains as Nominet is probably the worst of the bunch.
A good tip with these sort of things is to put content on the wesbsite and not just adsense adverts etc, at least you will have a good arguement should the worst come to the worst
