Comment Compaq TC1000, circa 2002 (Score 1) 201
The Compaq TC1000 and TC1100 were PCs running Windows XP Tablet Edition. They were just over a cm thick and had a nice tilt and swivel detachable keyboard. For the time, it was utterly astonishing and only really let down by its slow CPU and half-hearted support from Compaq/HP. Indeed I only switched back to laptops when HP discontinued the range. As far as form factors go though, it pretty much nailed how a hybrid tablet/laptop should function... ten years ago.
http://en.wikipedia.org/wiki/Compaq_TC1000