If it's something in the files it seems to be remarkably well embedded; I tried searching for the redirect code on the server itself but I'm not seeing any non-standard "header" commands in any of the files on the account; they're all the usual ones I'd see on a normal WP install. This doesn't mean it isn't in the files though, just that if it is, it's -really- well hidden
All I can recommend is what caftpx10 is recommending; if that fails, reinstall it from scratch. If -that- fails, then it's going to be from a plugin or theme, and I've got no real idea how to predict which one of those it is