i find it hard to be insulted by any attention, even if it's a bit overzealous. i'd personally be flattered, and tell them so, but then proceed to explain that it's waaaay too early to be bandying that word around (and yes, for effect, i may even say "bandying"!) if i did really dig them, i may wait to see their reaction to my explanation, to see how maturely they could take it, and whether they were capable of mellowing out. on the other hand, if i wasn't crazy about them, or was on the fence, i'd let them down easy by saying that i think we're in different places emotionally/relationship-wise, and that it wasn't going to work out.
but honestly, never would i feel insulted by a declaration of positive emotion. disappointed? frustrated? sure. it's difficult enough to find someone you'd even care to date in the first place.