That -erl ending is distinctively Austrian or Bavarian. You won't usually find it in Berlin or Hamburg! (Large parts of Austria speak a Bavarian dialect. Bavaria is a region in the Southeast of Germany.)
(I wonder whether the -el/ -l diminutive isn't a Southern German thing too, but I'm not sure. Maybe Satu or Rene know more.)
Nanni is a general German pet form of Anna/Anne and Marianne (though just as rare as Nannerl).
My name book tells me, French has Nannette as a pet form of Anne. Maybe that's how Nanni entered German - but I'm speculating!
(edited for clarity)
*****This message was edited 1/13/2007, 3:41 PM