Yes, there is an api and xapi (extended api).
I havent used these yet.
But there is also the cloudmade api to OSM. I played a bit with the python api.
I am not sure, if any of them could help you with your particular problem, but you could search for a coastline near to your point. If your point is within the polygon tagged natural=coastline, you have an island (or a continent)...
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