if you're looking for a good-specced ultralight, the main contenders are the lenovo x220, hp 2560p, panasonic s10, and sony z something-or-other (i don't think anyone else has mentioned the s10 which is why i am adding this post - see http://www.engadget.com/2011/08/10/panasonic-outs-toughbook-... for example).

if you're considering a macbook air then i suspect the s10 or sony are closest to what you want. personally, i'd go for one of the other two...