For the first time in 9 months or so since I got my 3gs, today I noticed a speak of dust under the glass. With in half hour of noticing it was annoying the he'll out of me. Went straight to my nearest apple store, well 30 miles away, but well worth it, after a little wait a brand new 3gs in my hands. Nice!
I noticed a speck in mine today, about the same age as yours. I'd get it replaced, but I don't want to lose my jailbreak. :\