This 360 has mouse & keyboard support! Yay! Except, sadly, it's not official support. Just the work of a modder. But hey, if he can do it, maybe you can do it too.
And if you can't, you can always buy it from him. XIM, the creator of the workaround, is selling the fix - which looks like it works by plugging your devices into a modified 360 controller - for US$180. Yup. US$180. That's steep, so steep it borders on extortion, but there's bound to be a sucker or three out there needing speedy mouse controls and faster text input that bad.