add an auto-increment column with rake?

Hi -

for some reason when I was learning Rails, I thought it would be a
good idea to drop the id from a table. I have no idea why I did it,
but now I'd like it back. I can roll all the way back, but I don't
want to lose my data if at all possible. Is there a way to add an
auto_increment column to a table, and secondly, can I do it and have
all of the id's automatically filled in on existing data?