Given a column of street addresses with city and state but no zip in a spreadsheet, I'd like to put a formula in a second column that yields the ZIP code. Do you know a way to do this?

I'm dealing with US addresses, but answers pertaining to other countries are interesting, too.

UPDATE: I guess I'm mostly hoping that there's a way to do this in Google Spreadsheets. I realize that you need to access a vast ZIP code database to do this, but it seems to me that such a database is already inside Google Maps. If I put an address in there without ZIP code, I get back an address with ZIP code. If Maps can do that lookup, maybe there's a way to make it happen in Spreadsheets, too.