This country has some damn fine Geeks, the technical team at NASA JPL have three working theories on what happened to Spirit.

a. The software which handles file management for the flash memory is not robust enough.
b. That there may be a problem with one of the motor control boards.
c. Or that a recent solar event caused problems writing to or reading from the flash memory.

A test done on the memory came back clean so software is very high on there suspect list. Luckily software can be patched hopefully they will narrow down the issue in short order and have Spirit back on the roll. [Spirit News Conference re-cap]