Actually, the previous MacBook Air 13" (2010) used a Core 2 Duo SL9400 or SL9600 which both have a TDP of 17W.
The new MacBook Air 13" (2011) uses an i5-2557M or i7-2677M which both have a TDP of 17W too.
I don't think we can say that these new CPU consume less power.
My personnal opinion is that Apple tried to stick to the power consumption while boosting the performance by switching to a more modern architecture. I do not expect Apple engineers to work on switching architecture AND improving power consumption at the same time.
So to answer your question:
No, I would not expect a longer battery life with these new CPUs. Even a little reduction for those with an 11" MacBook Air because their CPU switched from 10W to 17W.