If you think you're going to develop (read: compile) anything you might want to look at an Asus eeePC ... or buy a laptop cheap in Akihabara once you get to Japan. The Nokia Internet Tablets (especially the N810) would be fine for actually writing a little code. We have some nice text editors with syntax hilighting for at least a couple languages, but these little things don't have the kind of CPU, memory or storage space to host a real dev environment. The Asus eeePC isn't exactly pocketable but I can't think of device that really does what you want it to. Maybe a Kojinsha laptop? a little too big to be portable. A Fujitsu Loox FMV? pretty expensive.

Also: I don't think there are that many free public wifi spots in Japan. Unless you know there is some pay wifi in the area you'll be in or you plan on getting a bluetooth phone in Japan and pairing your device to it, you might not end up with Internet access most of the time. If you want to chat or send much email plan on hanging out at internet cafes.

For tons of bonus geek points: Pick up an N800 before you leave, then pick up a (used) Zaurus C1000 when you arrive in Japan. Install Debian on the Zaurus C1000 and load some compilers on it. I'm pretty sure you can find a used Zaurus for less than $200 in Akihabara or Den Den Town