A bit of grinding with a dremel tool and finishing off with a file took a small amount of metal off the back of the caliper. This gave me the clearance of a thick piece of paper - I'll need to watch to see if it's enough. Here's one arguement for using 17" wheels!

Post script to this story. (My camera broke so I didn't get any pictures). When I got the car running and on the road this clearance was inadequate. As long as I drove in a straight line it didn't rub too bad. But take any corner and the stresses/flexing involved would cause the wheel to scrape against the caliper. I ended up taking the rotors to a friend with a large lathe and taking a quarter of an inch off the diameter such that they are now down to 10 3/4". I then took off the caliper mount, filled in the mounting holes with my welder and re-drilled the holes such that the caliper was mounted 1/8" closer to the axle. This gives me a good 1/8" clearance between the caliper and the wheel. 400 miles of subsequent driving has shown no problem. Again - a warning against using 15" wheels with disc brakes - be prepared to do similar work to obtain the clearances necessary.