Odd. Asus has not updated their CPU compatibility list but they usually have an "AMD 6-core ready" bubble where the windows 7 bubble is on your board.http://www.asus.com/Search.aspx?SearchKey=M4A78%20PRO
The socket and chipset are compatible with the x6. A bios update for it will likely be released.
Asus 6-core motherboard page:http://event.asus.com/mb/2010/m4_6cores/