This is awesome! I have been looking forward to this! I love both American & Japanese music (although I admit that I'm kind of a bigger fan of japanese music since I grew up with it & watched alot of anime & japanese films).
I have been requesting Japanese songs from both Rock Band & Guitar Hero for a few weeks since getting the games. I feel that this is a fun mission to get these artists added. I'm happy to see progress but I'm still gonna request to show them some other artists as well as iocrease the fan base & the chances of these songs getting added. I recommend requesting, it's easy & even people who do not have the games can do it.
On Rock Band.com, the "request a song" option is at the bottom of the page and also shows up when you enter the "songs" section of the site.
On Guitar Hero.com, I usually search for something not on the site which pulls up the request option (haven'nt done it for awhile, I have been requesting more for Rock Band) I recommend this because the more people that we request, the more chances are these songs & bands will be added. Well, that's my suggestion to help bring Japanese music to Rock Band as well as give Japanese artists more exposure. Have A Nice Day Everyone! ^_^
P.S X Japan's I'V is already apart of Rock Band! It's featured as a free bonus track when someone purchase Rock Band 2 (gotta be new because the code used to redeem it would usually be used in a used game).