Easy option would be one that's already labeled as disc-compatible. Topeak has disc-compatible variations of most of their racks.

Less easy, but just as viable, is using a longer bolt on a regular rack and pushing the rack's legs outboard with washers and/or a spacer.

Could also go the other way and pick a bike frame that mounts its rear disc caliper inside the rear triangle, usually on the chainstay instead of on top of a seatstay. Quite a few are built like this -- Salsa's Vaya is one example:
http://salsacycles.com/bikes/vaya/

However, it's not a requirement, since Jamis's Aurora Elite has a seatstay-mounted brake and comes with a rear rack anyway:
http://www.jamisbikes.com/usa/thebik...roraelite.html

Since I'm posting links, here's one of Topeak's disc-compatible offerings:
http://topeak.com/products/Racks/Exp...ack_dsc_spring

... and some closeups of a Bikes Direct bike with a spaced-out regular rack:
http://www.bikesdirect.com/products/..._rack_gallery/

Anything else?