its just a place holder date until Square reveals the actual release date (sometime this summer). Amazon had Dec. 31 2010 because all they know is that the game will be released before the year is over.
i bet it wont be june if they don't have anything concrete yet. FFXIII took 3 months to localize w/ 2 - 3 entire SE teams. theres no way they are putting that much staff on localizing this (cuz they know it'll sell the same no matter when its released & doesn't hurt their bottom line unless it was outside their fiscal year). i know this isnt as big a game (content wise), but its darn big for a psp game. sucks cuz its in my quick picks today. if i thought it'd actually come out by july, i'd order it (w amazon cred. dont want to tie up my cred for an indefinite amt of time).
i saw a trailer for Birth by Sleep at Wal Mart but I don't think it had a date on it (but I didn't see all of it, only the end of it). Still no official date from Square but I've been hearing either June 29 or July 27. I'd think we'd get a date before E3 in june.