I am thinking of getting the new Kindle Paperwhite. My question has to do with the dictionaries. I reside in the US but I have friends in Japan who could send me a Japanese Kindle if need be. My preference would be to have a US kindle tied to my US Amazon account, as I do tend to buy ebooks from the American Amazon store. I'd be loading Japanese language ebooks through Calibre and probably won't be purchasing them through Amazon.

My question is does it matter if I have the US or Japanese version of the Kindle (or Amazon account) as far as which dictionaries I get? In my ideal scenario I would have a US kindle (tied to my US Amazon account) but could also have access to the J-J dictionary that normally comes with the Japanese Kindle, and would purchase a J-E dictionary based off JMDict. Is the J-J dictionary only available to Kindles purchased in Japan or can I install that as an American Kindle user?

Nope. You can download the dictionaries for any language no matter where you buy your Kindle. If you choose Japanese for the interface language it will download Japanese dictionaries by default (Daijisen J-J, Progressive E-J, and a monolingual English dictionary), if you pick English, it will download an English dictionary by default, but no matter what language you pick, you can download Daijisen & Progressive J-E later if need be.

There some some weirdness with the dictionary when you send files you created yourself to the kindle:

http://forum.koohii.com/showthread.php?tid=12062

The dictionary lookups for japanese text only work properly for files created by amazon (either files bought from amazon, or files converted by amazon through the personal document service thing where you email the file to an amazon address and they sync it to your kindle).
If you just send the files via Calibre (and they don't already contain the necessary information that splits up the text into words), you always select the full sentence when trying to look up a word.

Thanks for the replies all. It looks like I'll have to add an extra step at the end of sending my Japanese language ebooks through Amazon's service to get a properly tokenized version of the book.

I tested the Kindle Paperwhite in Germany and I could set the menu to japanese and got a J-J dictionary.

But the J-J dictionary actually really sucked- it doesn't select words properly, gives wrong definitions, wrong readings, and often it just gives you a word alternative (and you can't look up words in the lookup) like めちゃ as a definition of めっちゃ etc...
Also tested the reading quality of Kindle against a real book and the book was far easier to read..

I ended up giving it back because it sucked too hard - rikaisama is a thousand times as good...

I tried a while ago to get the Japanese dictionary on Kindle for PC and it didn't work, so it is not available for every version.

After struggling for a while trying to read an "innocent" book on my Paperwhite (with some success, but text selection was too often unpredictable), I finally gave it up and switched to the Wakaru app on iphone, which does a much better job despite the small screen. Highly recommended, btw