The head is on a spring steel suspension - lots of flex in the vertical direction, not much lateral. (Note: You've never popped open a drive before?)
Also, where the head touches down, they actually texture the surface of the disk to reduce the stiction effects and make the spinup easier.
As for the tracks, they are initially written out on a very precision machine, or may use some sort of fixture to externally move the head and write out the low level formatting info. Once they are written, the drive just rewrites these tracks, keeping the same position - these tracks are the only way the drive knows what the current lateral (and radial) position is.
If the drive gets bumped hard enough, and the head gets knocked off the current track, and if you happen to be unlucky enough, I'd imagine that the drive would mangle the data, possibly bad enough cause that part of the drive to get remapped to a spare sector. When reading, the drive will know that it got jostled off the track and will probably automatically reread it, as
long as the data didn't get physically scratched off.
Recently there was an auction where they were selling off equipment from Maxtor (it got acquired by Seagate), and there was a lot of drive related equipment being sold off. Interesting stuff.
http://www.hitachigst.com/tech/techlib.nsf/techdocs/9076679E3EE4003E86256FAB005825FB/$file/LoadUnload_white_paper_FINAL.pdf