Even if the the next iPhone fits in the cradle, that doesn't mean it will work unfortunately. There is a thread where someone modified a cradle to fit a new version but iDrive detected it wasn't compatible and rejected it.
It's taken almost a year for BMW to release the media adapter for the iPhone 4. Expect the same delay for the next iPhone. With this in mind, I recently got a 12 month contract for an iPhone 4 (rather than wait for 5)
BMW X3 (F25) xDrive20d SE . . titanium silver . . titanium front grille . . 309s . . xline . . xenons . . black leather . . alu trim . . professional multimedia package . . dynamic package . . climate package . . electric seats . . folding mirrors . . servotronic steering . . DAB radio . . business speakers . . sun protection glass . . split rear seats . .