Personally, my guess is that the iPhone will be somewhat popular among foreigners (especially Westerners) living in Japan, but not so popular among Japanese people. Even die-hard

Mac & iPod fans are mostly giving the iPhone a pass here.
If you read some of the early reports from the Japanese press, one of the things they are all angry about is the fact that the iPhone has handwriting recognition for Chinese characters when in Chinese language mode, but won't do it in Japanese mode (even though they use mostly the same characters). Japan and China are like fierce "sibling rivals," and giving a very useful feature to the Chinese but omitting it for the Japanese without any explanation is like a slap in the face to a lot of Japanese people.
The lack of MMS, which is heavily used by all the other Japanese celphone providers, as well as the lack of video calling (not super popular here but a fairly important checkbox feature to have), Felica/"Saifu Keitai" (aka Wallet Phone, RFID money card/train pass), and 1seg TV (mobile HD broadcast offered by all major TV networks), results in the iPhone comparing poorly to even the free throwaway phones you can get from any of the Japanese providers.