Cities to Metropolitan areas

I have a database of cities (in both the US and the rest of the world). The database entries are simple text strings. Currently the database contains even small cities (population 10,000). I wish to use a larger scale database instead, by assigning the cities into larger geographical\metropolitan areas. An example can be the Metropolitan areas in Google's AdWords API:

http://code.google.com/intl/iw-IL/apis/adwords/docs/developer/adwords_api_us_metros.html

but I have no problem u开发者_StackOverflow中文版sing other databases instead.

I'd be grateful for any advice on how this can be performed using existing databases\APIs!

You can use Yahoo's GeoPlanet database. You can easily find what's the parent of each place (WOEID - where on earth ID).

EDIT: You can also access this database using YQL.