Well, what have we here? No, seriously, what is this thing? Considering the absolute dearth of information given on GameStop's pre-order page, we're left to surmise that the Xbox 360 HDMI Conversion Kit exists (er, will exist) simply to allow those who purchased their Xbox 360 before the days of built-in HDMI to utilize one of the dusty HDMI sockets on their TV / receiver. Granted, we're sure hoping there's more to this thing than that, or else we're left wondering who's desperate enough to drop $89.99 to free up a single component input.
Update: Turns out this thing does indeed support 1080p, and it'll come with "free" HDMI and optical audio cables, too. Check out the PDF here. Thanks,