Originally Posted by fixedpip
A braking surface is just a section of the rim that lines up parallel to the brake pads. On older rims, the brake track is often not treated differently from the rest of the rim but machining them provides a high quality surface that improves brake performance. This is of course off the top of my head.
So yep you can get clinchers without a 'braking surface'. Some very aero rims and track rims don't have them, but its hard to find non-machined rims these days.
As Jobst correctly observed, the whole machined sidewall is really just a bunch of bullsh. It's purely cosmetic. It makes the bike sparkle on the floor, it prevents brake squeal on test rides, and it covers up uneven rim joints caused by riveting them. Any braking under wet or muddy conditions will get grit up on your rim and "machine" them for you. Whether grooved by machining or ordinary dirty braking, it *does* improve braking performance by providing channels for water shedding and exposing the base metal (versus the ano surface which is bad for braking) but machining artifically thins the rim walls which is completely unnecessary.