That's not much of a coincidence: The 911 is actually a fairly old design (early 1960s) and the follow up to the Porsche 356 [1] where the similarity is even more striking. They are also from a technical point of view fairly similar: Both have an air-cooled boxer engine in the back which certainly influences the design. There's also the "Berlin-Rom-Wagen" [2], a sports version of the beetle which was built for a planned long distance race from Berlin to Rome. It already looks very much like the Porsche 356. So the history of the two cars is actually quite mixed up.

[1] http://en.wikipedia.org/wiki/Porsche_356

[2] http://en.wikipedia.org/wiki/Porsche_64