I vaguely remember reading a blog entry (written by a well-known blogger I think) about how to model price changes over time, and that you could model most changes by saving two dates only (two columns in a db). The blog talked about prices on a website changing over time and how you could figure out the right price to charge knowing only when the purchase had been made.

Very vague, I know, but my google-fu is failing me, everyone at IRC are busy talking about other stuff and I don't know what to do! :)