Prior to the iPad, analysts probably lumped "tablet" sales in with generic PCs because, for the most part, they very much were generic PCs, like my old IBM X41 Tablet, which was just a regular subnotebook with a funky swiveling screen and a stylus.
Post iPad, and particularly with the attack of the clones coming soon, I expect analysts will treat "tablets" as a fully distinct category, to perhaps be defined as "too big to fit in your pocket, and lacking a conventional keyboard." On a definition like that, convertible Windows laptop/tablets would be "notebooks" while the rush of new iPad clones (and oddball gadgets like the Modbook) will be "tablets".
Categories aside, you do have to hand it to Apple for selling so many iPads. I'm still in the camp of "it doesn't do anything I can't already do just fine with what I have." Between my smartphone and my Kindle, I've got all of the iPad's bases covered. However, if my 91-year-old grandmother, who has never used a computer before in her life, came to me tomorrow and said "that's it, get me on the Internet", I'd have no hesitation in getting her an iPad.