I'm spoiled by Notational Velocity, which saves automatically, and by iPad apps for same. I have Coda and TextMate set to save when they lose focus so I don't lose work...
If your autosave makes you run out of disk space, you're probably "doing it wrong".
They are not so rare at this point in history - many people use a laptop as their main/only machine, and the move to SSD shrunk my disk by 3X. Space is going to be tight for a couple of years - I traded it for great performance.
Your computer already kind of does what you describe: That's what virtual memory paging is. And when your system starts paging a lot, it's noticeably slower. And when your computer starts paging with a nearly full disk, it is an out-and-out dog.
It's entirely viable for a filesystem to archive all of the data you've ever created. Treat the archived files as an LRU cache and, when new data needs to be saved, just write over the least recently used data. Instead of a volume bitmap of used blocks, you have a tree that can be traversed to get the blocks that can be overwritten. This is not data that must be saved, like the swap; it's data that should be saved until something else needs it, at which point it's overwritten, like most OS's RAM IO buffers.