*in starred states you can also register on election day, if you miss the mail-in deadline, which is also true for Montana and Wyoming, whose mail-in deadline has passed. North Dakota does not have voter registration, according to that reference, so I guess North Dakotans just walk in and vote.
Do register, and encourage others around you (or in states mentioned above) to do the same.
Oh, and if you haven't seen it yet, Fake Presidential Candidate Paris Hilton consults with famous fake president Martin Sheen for advice on her ongoing campaign platform.