[dougbrowne]

iPhone all the way!!! I have mine and it's great. There are TONS of Japanese apps that a very useful...

To name a few:
-Japanese: An awesome japanese dictionary
-iKanji Touch: A SRS Kanji learning app thats VERY effective.
-Human Japanese: A great app for learning the basics of Japanese

The iPhone plans in the US all come with unlimited data so you don't have to worry at all about data usage.. Also the phone has a full browser which can access any webpage you can on your computer (Execpt Flash sites) plus you've got an app store with tens of thousands of apps and quite a multitude of Japanese ones. Not only japanese learning apps, but you can also download japanese apps that would normally be sold to people in Japan to help immerse yourself.