Yeah but the P965 chipset was designed with Q6600 in mind as the quad. It MAY support Q9xxx series with updated BIOS but if its an early revision P965 you are limited to the 65nm chips.
Well, according to the specs on ASUS' site, that particular board is supposed to accommodate 45nm chips, and lists the Q9550 on its CPU support list:
http://asus.com/product.aspx?P_ID=bswT66IBSb2rEWNa
Although you're right -- it's certainly worth double- and triple-checking that kind of thing, because it's pretty lucky for a board that old to support modern-day hardware. You definitely made me go back and look at it again.
There may be something I missed, but it sounds to me like when they put together this particular board, they meant to market it as "compatible with any CPU that comes out in the next two years" or something to that effect, and it might have actually turned out that way.