"This is a new feature in Battlefield 3 where lightly damaged vehicles recover their armor after a set amount of time (if kept away from enemy fire). Instead of promoting a gung-ho attitude at all times, this means that the driver who keeps his cool and adapts to any given situation will also be able to keep his vehicle in working order for longer." read the blog.
However, if a vehicle is heavily damaged, it'll require repairs from an engineer. Additionally, while the vehicle may be in flames and immobile, its weaponry will remain operational. According to Battlefield 3 lead vehicle designer Patrick O'Shaughnessy, "In our daily playtests here at DICE, you will often find a disabled tank in the field with Engineers scrambling to repair it, while its crew is still operating the main cannon and auxiliary machine gun to keep enemy forces at bay. If repairs go well, the tank gets back to full working order and the crew has narrowly escaped a disaster. If not, at least they had a fighting chance, while the disabled vehicle itself made a tempting target leading to more intense multiplayer action."
Battlefield 3 will include 20 different driveable vehicles, including scout helicopters and fighter jets, as well as over 80 vehicle upgrades to unlock.