There's a great article at The Guardian today that compares the iPad's introduction to that of the iPod and iPhone, and offers interesting numbers regarding Apple's share of the tablet market.
Last January, IDC analyst David Daoud projected that sales of non-Apple tablet PCs will fall to 1.25 million units in 2010 (down from 1.32 million in 2008). At the same time, projected iPad sales for 2010 range from Piper Jaffray's estimate of 4.3 million to Forrester's 3 million (iSuppli says 7 million, but that seems a bit nuts).
Let's say they're all way off and go with a conservative guess of 2 million units sold. Even that number would make Apple the largest tablet computer manufacturer in the world. Already.