Brian,
Although the following information retrieved from IGN was (I belive) later disconfirmed, this is what I was able to find.
"Though the information is not available via their websites, major videogame retailers, including Gamestop, list an Xbox version of the game with a March 2003 date in their computer systems. Phone calls to various chains around the San Francisco area have confirmed this appearance in multiple stores.
Officially Konami hasn't released a statement regarding an Xbox version of the game, and there are no materials other than the listing in the aforementioned computer systems confirming its existence. However, retailers enter this kind of information into their databases whenever they are given a SKU number for a game, meaning the game is almost certainly a reality."
I am obviously taking all unconfirmed reports with a grain of salt. Especially since it's now April 2003.

Hm, time will tell I guess.
~j