Based on my research, I'm tentatively concluding that such a device does not
yet exist.  But even assuming I'm right, it's probably safe to say that it
will soon.

I've seen cell phone models with a touchpad, and I've read about models with
a built-in Japanese / English dictionary.  Obviously, a "Smartphone" would
potentially be a very flexible platform for non-commercial development.

Currently, I use a PocketPC2002-based Japanese PDA with JWPCE installed, and
frankly there is no going back once that magical solution is made possible.
But even a small PDA like my Toshiba is a trifle bulky, and I'd be more than
happy to merge this functionality into a smaller device like a cell phone,
especially if doing so simultaneously renders one handheld device redundant.
It would also give me an excuse to finally drop-kick Southwestern Bell.

So, has anyone heard anything about any upcoming cell phone models which may
enable handwritten kanji/kana input, in a manner not unlike what one may
typically find on a Japanese PDA?  Granted, cell phone display resolution
may not be up to the task yet (220x176 is the norm, I believe, which puts it
neck and neck with a Gameboy Advance), but I doubt it will remain so forever.
And it may in fact be adequate.  Obviously, the thing I'd most like to hear
is that JWPCE can and will be ported to a cell phone in the near future.  But
for now, that seems to be a distant hope.