Hmm, you could certainly allow the users to enter text in the drop-down field and then compare the first 3 (or all) characters against your data by looping over it. I doubt though that that'd work in real time, e.g. provide a user experience as in a Flash/Flex application where the number of options...

I've seen that question popping up a few times here and there and as far as I know there's unfortunately no way of doing this.CheersKai