If you get a new 2009 MacPro and the new 24" display (you know, mini-displayport), you will first need to use a DVI monitor to get past the initial setup--just spent an hour on the phone with Apple, who didn't know this (only tests with up-and-running machines, not wiped-for-customer-out-of-box ones)--was my suggestion to try this (end-user pickup up Apple's slack?!--poor planning Apple).
I know, it doesn't seem right, but I'm sure even a VGA (with DVI adapter) monitor would work--just need it to the regular once-you-are-logged-in, then you can just hot swap for your one or two 24" Apple monitors (that look sexy, but again: BEWARE THE SHORT CORD).