A pilot reported that he became distracted while navigating with an iPad and failed to realized he had entered class D airspace until he visually sighted the airport.

Narrative

[Was in] level cruise at 1;500 FT MSL heading NE. Had [a] paper sectional chart and new iPad with scanned sectional chart (both current). Was using ATC for SFRA services and was terminated north of BWI. Began to use iPad for navigation; but had it 'zoomed in' too far and lost track of the big picture. Saw MTN visually about 3 NM from my position and realized I had to be within their airspace and I didn't even have the frequency to call. I turned away; watched for traffic; and when well clear of traffic; I looked up the Tower frequency and radioed to make sure there had been no conflict. Controller was very professional and fair; reminding me of my obligation to contact the Tower and the fact that he had a formation flight inbound whose path I crossed under. Hind sight says that I should have converted my SFRA radar service into flight following. Obviously; I [should have] spent more time consulting the real sectional chart; or even my moving map GPS instead of becoming distracted with the new gadget for the information. The shape of the class D airspace northwest of MTN is not circular and I did not notice the straight edge of the airspace on the moving map until too late. A more common circular dashed line would have immediately perked up my suspicion; prior to entry.