It finally arrived!
Since my Sony PRS-T1 was Japanese-only, I was first of all very interested in whether the user interface language could be customized. The PW-J lets you set the UI to 9 languages: German, English-UK, English-US, Spanish, French, Italian, Japanese, Portugese-Brazil, and Simplified Chinese. There's no option for Traditional Chinese, which seems odd. The same 9 languages get keyboards.
English has two dictionaries: The New Oxford American Dictionary (English-to-English), and the Progressive Eiwa Chuujiten English-to-Japanese dictionary. Japanese has only one dictionary, the Digital Daijisen. Additionally you can download these dictionaries from the cloud: the Oxford Dictionary of English, Portugese, French, German, Spanish, Italian, English-Chinese, and what I believe is Chinese-to-Chinese.
For Japanese books there are 2 font choices, one kaku-gothic (think the equivalent of sans-serif) and one minchou (think serif). Interestingly enough, when you open an English book, you instead get the 6 fonts that non-Japanese PWs have. I'm impressed! (My Japanese PRS-T1 has one font. One. Not impressed, Sony!)
Japanese text is very prettily displayed vertically, with page taps going forward to the left just like you'd expect. The kanji pronunciation guides ("rubi") are displayed when available, and look great. A very good job.
As of today the Japanese Kindle store has 1,548,195 books, of which 1,475,772 are apparently western books, for a grand total of 72,423 Japanese books in all genres. Well they're working on it, maybe the Kindle will be the one to open up the Japanese ebook market...
I bought some free books and successfully transferred the AZW3 files into calibre. The three onboard dictionaries failed to convert, but the Japanese user's guide and Welcome letter transferred fine. I'm guessing the free books weren't DRMed, and I haven't verified that my settings for the relevant plugins are correct, so there's more experimentation to do there.
I then side-loaded the AZW3s onto my Kindle 3/KK (after informing calibre that the KK likes AZW3 format in its settings panel).
The KK displays the Japanese text horizontally instead of vertically, but the AZW3 format is still smart enough to display the rubi above their words. So that's nice. At large font sizes the rubi get scrunched into their words, and at tiny sizes they're too far away, but this is still perfectly readable, heroically so for a non-Japanese machine. (I also tried converting the AZW3 to the old mobi format, but the rubi are then all just confusingly shuffled into the text so that's not a realistic option; AZW3 it is.)
Perhaps surprisingly, there seem to be two japanese fonts available on the American KK, an acceptable maru-gothic (sans-serif-ey, but rounded) and a ratty minchou (see the screenshots). I'll happily read with the maru-gothic font.
Converting the Japanese AZW3 to ePub worked fine, and opening this up in Sigil revealed that the vertical text is apparently part of the way they've got the CSS set up in the file...except that the characters are all rotated clockwise...there's a screenshot of this too. More experimentation to do here as well.
I used an online conversion tool to convert classic public-domain texts to ePubs, then loaded them onto both the PW-J and KK as AZW3. Although the texts have the rubi embedded within the text, the conversion tool plucks them out and reformats them into the correct places above their respective words. These files worked fine though their text was horizontal on both devices. Presumably more CSS fiddling will eventually take care of that. calibre plugin anyone?
The conversion tool I used is here:
http://naoki.sato.name/epubunco/bookmarklet.html
So... It looks good! And I've got a lot of playing around to do!