That is, I have information in column B, and I'd like a datestamp in column C noting when column B was filled out. I see a bunch of onEdit scripts, but I'd like it to be retroactive--those cells have already been filled out. Is that possible? Does Google Spreadsheets store that information and is it accessible?
Unfortunately, I don't believe it is. As you mention, there are ways to get the timestamp going forward, but the information as to when a cell was filled out is not retroactively available.
The API doucmentation provides an updated field on List and Cells feeds, but that is a sheet-level property. The returned cell data does not contain any other information regarding the date, so the (unfortunate) answer is that you will likely only be able to get that level of information going forward.