Take a look at the Cove Stiffe FR. I have one and love it, I take it to the races and use it as a backup bike, and on some courses it's my first choice. I bought mine used on RM but new frames are under $500 at
Jenson. It has disc bosses, I am running V's on mine now but after the new xc frame gets here I'm putting my dh wheels and discs on it and turning it into a ghetto bike. I have Hayes and 6" Shimano discs, Hayes makes adapters to go from either IS to 6" or IS to 8" and you can use any brand of disc with the same caliper, all you need is the proper adapter (I have tried it and it works).