Implies that in the US you can turn up without being registered and vote. In the UK you can't do that, you have to be registered ahead of time to be able to vote.

Well, basically you have to sign in on the registry. If you are not registered, you are allowed to vote "provisionally", where you cast your ballot, but they put it in for special handling. After they verify that you are qualified, they then count your vote.

Some good information on US voting registration can be found on MTV's "Rock The Vote" web site. MTV has been running campaigns to get young people registered and voting for a number of years now, and they have some good information on their site.