Can't wait to see Up? The only thing you can do now is wait. And reserve your tickets for opening day! In line with other recent Disney releases, the official Up website now has a "Get Tickets" feature (for North American residents only) which allows the public to purchase tickets in advance.
Punch in your ZIP/postal code or city and state/province to get a list of local theatres and showtimes. Click "Purchase Tickets" to be connected to the theatre's website where you'll buy the tickets using your credit card (or debit for some theatres).
I tried this when buying tickets to Disneynature's Earth (which was amazing by the way) and it's pretty good. No having stand in line at the megaplex.