According to Philip Elmer-DeWitt, the general consensus is that Apple has sold about 8.25 million iPads. He's gathered quarterly estimates from many sources and compiled a list. They range from 3.80 million iPads sold (issued on September 28th by Brian Marshall of Gleacher & Co.) to 6.0 million (issued on October 1st by Robert Paul Leitao of Apple Finance Board).
As Philip points out, Apple sold 3.27 million iPads in the third quarter. Apple will hold its 4th quarter financial earnings conference call on October 18th, and TUAW will be live-blogging the event. Check back then to get the real numbers.