I'm disappointed in the Leona Lewis announcement too. I'm not crazy about using popular comtemporary artists for video game music (English versions). Not that I'm being racist/stereotype or anything, I just think it takes away from the game.
In the case with Gackt or Angela Aki, for some reason I don't find it quite as detrimental because Jpop artists are regularly used for anime/movie/video game songs.
In particular, I love it when composers choose a not-as-well known artist who ends up being an amazing singer. A good example would be using Susan Calloway for the Distant Worlds release of Distant Worlds, and she did such an amazing job that she's going to perform a whole knew selection of FF songs, eg Melodies of Life, Suteki da Ne, etc
Leona Lewis is a pretty amazing singer, much better than some of the other artists out there, but I'm not crazy about having her sing for a game I've been anticipating for so long. It'll make it like, a-game-Leona-Lewis-sang-for. Not to mention her style in regard to video games is...meh.