P18F4550:
Obsticle avoidance is quite easy, A rotating mirror, an infra red laser projects a beam, the mirror creats a horozontal line at a specific distance in front of the vehicle, a camera mounded above the laser looks at the terrain ahead useing edge detection the laser line will stand out from the rest of the image and obsticles will apear as breaks in the line.
Easier to build (may or may not be cheaper depending on what you have on hand, though) would be to use a laser module with a built-in line diffraction grating. Such a grating is also sold separately (if you already have the laser, for instance).
P18F4550:
Everything is available to the homebrewer, scanning mirror from old photocopier or printer, Gameboy cameras have built in edge detection and are easy to use
...but finding a gameboy camera is starting to get difficult; a better method might be to use something like the Nootropics Design Video Experimenter shield (or a similar circuit) and a composite video camera (aka - a common small security "spy" camera).
Personally, I think his array of IR sensors might be enough for short distances, provided the vehicle doesn't move to quickly...