I like the Samsung 13.3" Series 9 laptop at 2.6 pounds weight. But it has to have Win 7 and a regular screen because if a laptop is held and used like a tablet then the keyboard is just in the way.
I would be okay with a version of the Series 9 that rotates the screen and then snaps it into place over the keyboard with the screen facing up for tablet use. That could have Win 8 but it must not weigh more than 2.6 pounds weight with a 13.3" screen.
These convertible laptops that weigh 3.5 pounds are funny.
The other form factor is the Samsung ATIV 500T and 700T. They simply detach from the keyboard for use as a tablet. The weight is 1.6 pounds and the screen size is 11.6".