I've recently started something like this, but so far it's in German only, and only recognizes zip codes from Germany, Austria and Switzerland (it does a proximity search, not strict search by city).

The main problem is getting the data. I thought that ticket affiliate programs would give me the data in machine readable form, but so far no luck (one of them does, but it's far too expensive).

In case you're still interested, it's http://konzertgeek.de/ (source at https://github.com/moritz/soonish-p5 ).

If anybody has ideas where to get the data from, please tell me!