Akuma is a "sort of" boss character in Street Fighter IV. He'll turn up when you're kicking too much arse, and need a reminder on the natural order of things. Since late last week, though, he's also been a playable character, with a code distributed to Japanese arcades allowing him to be unlocked and played. The official site's even got his moves list up, so you can study up before hitting the arcades. As for SFIV cabinets outside Japan, well, this is what Capcom means when they say the imported units are officially "unsupported". Sorry!
「いよいよ、豪鬼が？！」 プロジェクトマネージャー：塩沢夏希 [SFIV Blog, thanks Marcus!]