I first read about this on Sheldon Brown's website, but he doesn't show how to build them. This instructable fills that gap.
Please note - If you build them and your brakes fail, don't come after me. Your bike was built to use a certain size wheel with a certain size brake. Change these things at your own risk.
A note about wheel sizes: I am using a set of 26" mountain bike wheels on a frame designed for 27" road wheels. The long-reach brakes shown here will reach a set of 700c wheels from the stock mounting location, but I didn't have another "spare" set of 700c wheels lying around. I did, however, have a set of mountain bike wheels. This bike is primarily used for a trainer so I don't have to mess with taking my favorite road bike off the trainer every time I want to go for a ride in the real world. The smaller wheels drop the bottom bracket and increase the risk of striking a pedal on the ground during a turn. If you mount a set of smaller wheels, always be aware of this when making turns.