The development has been widely reported in the Mac media and was also noted by The Register after Apple applied for a European patent last year.
Apple has long had some of the key technologies for a tablet computer, with Inkwell, a handwriting recognition feature having been in Mac OS X for some time. Still, Apple has thus far downplayed the idea of tablet PCs.
Of course, Apple has been known to downplay something right up until it introduces its own product. I seem to remember Steve Jobs talking about flash music players as throwaways only months before the iPod Shuffle was announced.