Where does Lion's autosave feature save past versions?
There are a couple SQLite databases located in a hidden folder on the root of your hard drive that are tasked with tracking the files and the differences between the versions.

The directory is:


/.DocumentRevisions-V100


The two databases are:


/.DocumentRevisions-V100/db-V1/db.sqlite
/.DocumentRevisions-V100/.cs/ChunkStoreDatabase


Source: http://arstechnica.com/apple/reviews/2011/07/mac-os-x-10-7.ars/14#versioning-internals