Of all the things that I'd think a UA would cache it would be a favicon. One useragens seems to request it every 5 to 10 minutes
Safari5531.9 CFNetwork/438.14 Darwin/9.8.0 (i386) (MacBookPro5%2C1)
I will assume that this is a MacBookPro 5.1 running Safari 4.02, but what does is it doing with that favicon that it has to request it so often?