My 2007 Specialized Tranistion Pro was stolen a couple of weeks ago so this is my "new" bike. I bought the bike minus wheels from another STer.

The rear wheel is a homemade disc converted from a Mavic Aksium training wheel. I cut the discs from flat ABS plastic sheets and bonded them to the wheel with Spray foam insulation. And yes it does make the ominous rumbling sound just like a "real" disc.

http://i39.tinypic.com/97l74w.jpg