My post got trimmed... the press release brings the date of non-IMAX releases up (to Thursday) and this does make the Wednesday advance IMAX "exclusivity" a bit less compelling... at the same time it hardly matters (as also mentioned in the press release) since the IMAX shows were mostly sold out anyway.
I did a search for Wed May 15th for the 10036 zip code and got 6 Imax theaters and one Regal RPX.
It's not opening everywhere on the 15th and I don't see anything in IMAX besides 8pm and 11pm shows on Wed. I have to believe that besides those Sneak previews on Wed. IMAX showings start the next night like everywhere else.
Winters: If Imax does not have an exclusive then it is opening everywhere. The funny thing is that Box Office Mojo and The Hollywood Stock Exchange still have the opening date as the 17th. This suggests to me that the studio got nervous about the early Imax opening at the last minute. They may be worried about negative word of mouth from early Imax viewers.