If you are going to launch this, can I suggest that, rather than IP detection, you build out a series of pages for each location, e.g. New York City, London etc.

That way you can get SEO traffic (London has a non-ajax, SE crawlable page), and also provide people a better experience when they plan to go somewhere (e.g. London from NYC).

Lastly - revenue model? I assume affiliates for hotels etc? If so, you need traffic, so see my first point :)