Rear brake pad clearance problem on '06 Specialized Langster
Well, I went ahead and ordered some Shimano 105 dual pivot calipers. Unfortunately the rear caliper arms will not clear the seat stays. There's just no fixing it as far as I can tell. If I put on additional washers, the fixing nut will not thread onto the main mounting bolt far enough. I tried several sizes of fixing nuts and none of them will work.

I hope that I will be able to find some other brakes calipers that will work. If you know of any, please let me know!

***Update: I solved the problem by ordering a SRAM Force brake caliper set. Those calipers have a lot more clearance between their arms and the frame, and the bike shop filed down the main fixing nut so it will fit inside the rear brake bridge.