Hmm, I use a Lenovo X220 convertible as a tablet for extended periods with little fatigue. It weighs 3.88 lbs. I've also used the IBM T60 tablet.

I'll admit that the iPad's lightness is a nice feature, but not a necessity. Also, I am not a power lifter, and for comparison my little sister has also used these form factors and carries a 17" MacBook Pro at 6.66 lbs.

My point is that for some people this is a viable form factor.

My primary concern about the Surface Pro is cooling since it doesn't appear to have fans and the last batch of Ivy Bridge CPUs were using some questionable heat compound between the die and outer casing. I also wonder if it will be capable of delivering reasonable performance with applications such as Photoshop or run a WAMP server and IDE at the same time.