well there's a lot more stuff being logged than just that, it just depends on how deep you want to poke. for instance, it has to know whether a thread has been "read" or not, for every user, even, dunno if there's a timestamp attached to that, it saves a lot of things (dunno for how long) in order to be able to present the information shown in the "who's online now" screen, and additionally there are the apache logs that I could grep for thread ID 23309 in the timeslot between 11am and 11.30 (my time).
however, I dont have much time anymore this afternoon and I'm off to the land of crappy internets this weekend, so it's a moot point now.