At present, ultrabooks use a ultra-low-voltage Ivy Bridge chip with a 17W TDP, while standard laptops feature a 35W TDP processor.
Mind you, a 10W TDP probably still might not be enough to build fanless Haswell tablet designs quite as thin as the latest crop of Android slates, because most ARM chips are more efficient still, but it could bridge the gap between Intel's Atom and more capable processors. Still, Intel reps seemed to suggest that we shouldn't expect the full capabilities of a 17W Ivy Bridge chip at the 10W level, so we're curious to see how Haswell will perform.Source: The Verge