This (via) is, uhm, telling:

Using Flurry Analytics, the company identified approximately 50 devices that match the characteristics of Apple’s rumored tablet device. Because Flurry could reliably “place” these devices geographically on Apple’s Cupertino campus, we have a fair level of confidence that we are observing a group of pre-release tablets in testing. Testing of this device increased dramatically in January, with observed signs of life as early as October of last year.