Setting a cookie when the user updates her preferences in the db, and then checking for the existence of it before hitting the db if it doesn't... Or maybe the extra cost of hitting the db just isn't that high with todays hardware(?)

In case you're going to use cookies it might be a good thing to add a user_id or some other token to the cookie name, in order for multiple persons to be able to use the same browser; "acs_prefs_$user_id: ...." (IE works like this anyway)

Just my tuppence worth,