The problem I'm having is that this has been going on for quite a while now. Every two to three months, we've had to reset the map, and it gets really annoying. We're just curious as to why it seems to happen so regularly. Is it just because it's in Beta has is bound to have loads of bugs anyway? Or could it be hard drive related?
Hard to say really, there are so many files Minecraft writes to record everything in the map that if even just one is messed up the whole thing will be screwy. It could've had an improper shutdown, a file could have been written incorrectly or incompletely or a handful of other things. Your harddrive may be having issues storing the files also, but I can't really say for sure.
Your best bet is to do regular (daily at minimum) backups, that way if something does happen you can roll back just one or two days instead of losing months of work.
Every instance of a world being corrupt in Minecraft has been solved for me by replacing the level.dat, level_old.dat, and session.lock files in the world directory with ones from a backup. Something happens when Minecraft crashes and those normally 126 byte files become 0 bytes. Then Minecraft can't load the world.
They are absolutely required for the map to work. From my understanding, they are the seed to the random generator that builds and loads your map. If you lose that seed, Minecraft doesn't know what to do with all your world data.