Update: Square Enix has told us this was actually confirmed last year (whoops), and here's a copy of the relevant statement on the issue:
"Just Cause 2 is an incredible looking title with one of the most richly detailed landscapes ever seen in a computer game, and to achieve this on the PC, the minimum specification includes a DirectX 10-compatible graphics card and operating system. The DirectX 10 and 10.1 APIs have allowed us to achieve an unparalleled level of visual quality, adding features and routines that simply would not be possible under DirectX 9. As a result, the graphical and performance gains have allowed us to create a game which raises the bar significantly in PC gaming and offers the player one of the most immersive and enjoyable gaming experiences."
Original story: Square Enix has said that Just Cause 2 will not support Windows XP.
"Yes that is true... there is no XP support for Just Cause 2," community manager Mike Oldman told VE3D.
"This is because the game has been written to take advantage of the extra performance offered by DirectX 10 and sadly Windows XP does not support that application."
Our friends at Rock, Paper, Shotgun responded by noting stats from Steam's latest survey, which suggest 42.15 per cent of gamers still use XP.
We've asked Square Enix if it has anything else to say on the matter.
Just Cause 2 is due out for PC, PS3 and Xbox 360 on 26th March.