>>I see Wikipedia places the legend about it being from Hamburg, NY on top. But then, assuming that the 90% of hamburgers are consumed in US-based establishments, we can safely assume that 90% of the Wikipedia article writers are of the same origin. At least there is a proud mention of pljeskavica, which then leads to http://en.wikipedia.org/wiki/%C4%86evapi which then means I'll be hungry early today, having seen the article :).
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>On the radio a few weeks ago, they said that the hamburger was invented on of the emigration ships from Europe to USA. Since the fresh meat had become bad, and they only had salted meat, one of the chefs tried to mix the salted meat with other ingredients. The result was very popular, especially after he added two piece of bread, on below and one above the meatball.

Accidentally, "pljeskavica" (pattie) means something flattened by hand clapping. Its nickname "pljesak" actually means applause. Maybe that cook got the idea after getting one for the first round meatballs? :)