iPad has bigger battery (42.5Wh) compared to 11" Macbook Air (35Wh). I have no problems with Macbook Air's battery life. Thus Intel's recent (Sandy Bridge) CPUs already work fine with small batteries, and the battery life will only improve with Ivy Bridge. This is especially true if Ivy Bridge's configurable TDP is used. When a lower 13W configurable TDP is used, Ivy is still comparable to older 17W Sandy Bridge laptops in performance (Anandtech benchmarks).
The only thing I dislike in x86 Microsoft Surface is the weight. It's weights 43 grams more than the (last year's) Samsung Series 7 Slate, and has similar specs... Except the older Sandy Bridge CPU is of course replaced with the more recent Ivy Bridge (but that should help with the power consumption and thus reduce the device size instead of increase it). They keyboard "smart cover" is a nice new innovation, but I am not 100% confident it, or the fatter "type cover" will offer comparable productivity to the Samsung's excellent chicklet keyboard (that is 1:1 carbon copy of Apple's small form factor keyboards). Competition is of course always a good thing, but I will personally wait to see what other manufacturers have in their sleeves for Win8 launch.
It's entirely possible that Samsung can shave 60+ grams of their design, and thus have a 100+ gram lighter Ivy Bridge based competitor with identical specs. Just look what they have achieved with Galaxy S III (133g, 8.6mm). It has almost twice the display area of iPhone 4S (140g, 9.3mm) but weights less and is thinner. Surface is an excellent first try from Microsoft, but I am not yet convinced it will be the best Win8 tablet during the launch period.