I know this is an old thread - these days, you can actually order laptops and iMacs from the US Apple store with Japanese keyboards.
Just in case some people are wondering, even though it's true that one can type romaji in Japanese on any keyboard (this is the fastest form of entry for touch typists), there are convenient かな and 英数 keys on the bottom for automatically switching to and from Japanese and English modes: very nice!
Even better, if you are a programmer like me, there is a control key where the (useless) caps-lock key is, on the left-hand side for your pinky. If you are an avid user of emacs or vim, having the control key in that position can actually save you from getting carpal-tunnel!
For these reasons and for touch typing efficiency (some punctuation marks are in different places compared to the US keyboard), all my Apple laptops and desktops use Japanese-version keyboards. I intend to keep it the same for future purchases.