Same day registration is horrible. I've worked the polls every election in Connecticut for the past 5 years, and it would just be a disaster. If you really want to vote, why don't you just go ahead and make sure that you are registered before the election.
No, how about the government automatically registers you like Ernest suggested. Then it's up to you whether you want to vote but it won't depend on whether you remembered to register within 15 days before the election. It's not a big deal for us junkies, but normal people aren't always thinking about elections and in many cases don't even know about these laws.