So, today I decided to start a new character, a Dunmer Mage. I load up a new game, and about an hour later I'm in Whiterun. Deciding to switch character, I quit out and go to load my main - a 120-hour, lvl 41 Nord warrior. Thing is, I can only find my current save file. After scouring every last inch of my PS3's harddrive, I figure out that the autosave saved over my file.

Well, there goes 3 weeks of my life. I think I'm gonna take a break from Skyrim for a while....

an autosave saved over your main save file?

as far as i'm aware autosaves are files on there own and cannot (and will not) save over an actual proper Save file.

sure you didnt accidently save over it yourself?.

on the upside i suppose 1.4 is out for you soon (if its not all ready?) starting a new game on the PS3 will probably be the best option anyway with the memory fixes.

Ah well. I did managed to break the Thieves' Guild and Forsworn questlines on that char. I also got J'zargo and Meeko killed last night....

It's a known issue. When you select New Game and after the cart ride scene finishes the game makes a save.. It's not marked as auto-save, so it's possible to manualy overwrite it. When you start new game again, skyrim will claim the same slot and write over it. A way to avoid this would have been to keep more than 1 save and never save over that first save which the game creates.
it's there so you dont have to watch the whole cart ride again when you start a fresh character, but the way beth did this was a bad move.. should have been labeled as Auto-save.

You'd think I'd learn to turn autosave off completely...

Those saves are scripted and can't prevent that one.