Kevin C. Tofel asks on GigaOM what problem Intel's Ultrabooks intend to solve, and asserts its to solve Intel's problem. Worth a read.
For me, an ultrabook would fill the niche between my souped up desktop at home, and my smartphone. As a "power user" and developer with very specialised software requirements, I need the lightest possible machine with decent battery life, a full sized keyboard and the ability to run desktop applications. My back can't take carrying a 15 inch MacBook Pro any more. iPads and Kindle Fires can't fill that roll, nor are they intended to.
The problem for Intel, and I think this fits into Kevin's argument, is that people like me (and perhaps you, if you read this blog!) are in the gradually shrinking minority.