I get that a lot. Black screen crash, same position and inventory, but all unsaved chunks revert to their last saved state. I get around it by bringing up the menu (which saves the map) now and then, so that it's always got a recent saved state. Damn annoying, though. Especially when mining. Spawning in solid rock isn't fun. (So if you get the problem, always, always, save before entering a newly excavated space, I cannot stress this enough)
Honestly, I'm really waiting for the Nether to be made more useful. Yeah, the space compression is nice, but aside from that there's very little motivation to do anything, and once something nice is added, all the old chunks won't have it (like perhaps a greater abundance (than in realspace, since none are found there now) of ores in the netherack, making mining operations there worthwhile). I suppose by then there'd be motivation enough to regen a new world, and just deleting the Nether folder from a world would mean it could be regenned with any new features, but still... I suppose there's not much reason to avoid it, aside from the horrible noises and the constant threat of horrible death by Ghast (who make it almost impossible to do anything on the surface, since they spawn in ever greater numbers until there's a sea of fireballs everywhere)...