I also have old worlds from Beta 1.2 which I saved as singleplayer worlds with the standalone client. Do you happen to have any worlds saved from the Classic 0.30 version which was played on the website: play.minecraft.net ? In Feb 2011, I played the free demo of Classic 0.30 first before buying the game. I dug through the browser cache folder and found the "level.mine" files, which are essentially the entire world from 0.30. These were then loaded by me years later into MCedit, and converted into McRegion files, which I could load into Beta 1.7.3. I was able to play the old website worlds again ! Obviously, the main differences would be in the lighting engine, and basic things such as grass spreading, and fluid mechanics, but otherwise, all of the blocks loaded just fine with no issues.
