I'd say it's probably pretty high. Probably close to 100%. I would guess setting a premiere up was the main reason for the delay in the announcement.
That's my assumption too. But I thought of a few problems. If it's a month before the normal release I think it's safe to assume this would pretty much be the world premiere (given the location that makes a bit of sense too). I just wonder how logistically they're going to fit everyone from Botcon into a cinema. My only solution was some kind of raffle system similar to what they did for the Hasbro tours at Botcon '07. Which would freaking suck if you missed out. Does this sound absurd?
I would guess that they would do it a lot like they did at the movie showing in 07'. They rented out the whole theater, and had many screens playing it. If I remember correctly it cost something like $20, and that was donated to charity.
I to imagine it will be a similar set up to 2007 as for the premiere thing it really doesn't mean anything. It could be classed as a test screening. Crews generally get a screening prior to the premiere press have screening early to so its not likely anyone will care if we see it at the end of May.
I have not check for myself on the tf club boards but It was confirmed that we will see a early cut of the moive It may not be the one that hit theather later.
I just ran through all the recent club threads, and didn't see it anywhere. The closest I can even find that might be taken that way is on the boards here.