I believe that the Flickr API won't return any results if you don't put additional search terms in your query. If I recall from the documentation, this is treated as an unbounded search. Here is a quote from the documentation:
Geo queries require some sort of limiting agent in order to prevent the database from crying. This is basically like the check against "parameterless searches" for queries without a geo component.
A tag, for instance, is considered a limiting agent as are user defined min_date_taken and min_date_upload parameters — If no limiting factor is passed we return only photos added in the last 12 hours (though we may extend the limit in the future).
My app uses the same kind of geo searching so what I do is put in an additional search term of the minimum date taken, like so:
http://api.flickr.com/services/rest/?method=flickr.photos.search&media=photo&api_key=KEY_HERE&has_geo=1&extras=geo&bbox=0,0,180,90&min_taken_date=2005-01-01 00:00:00
Oh, and don't forget to sign your request and fill in the api_sig field. My experience is that the geo based searches don't behave consistently unless you attach your api_key and sign your search. For example, I would sometimes get search results and then later with the same search get no images when I didn't sign my query.