Wikipedia API for geolocations

UPDATED ANSWER:
Wikilocation has been retired and now there is an OFFICIAL WIKIPEDIA API

? action=query
& list=geosearch
& gsradius=<radius-in-meters>
& gscoord=<lat>|<lon>

HTML example | JSON Example

Don't reinvent the wheel, use DBpedia.
Sample page with "lat" and "lon" data extracted.
And it can be queried with SPARQL.