"""Is Thunderbolt really faster than PCI-e?"""
Technically, I think Thunderbolt runs PCI-e (4x?) over the cable, along with some other things. I imagine latency is higher, but it should be enough to drive the sort of GPU you'd expect in a reasonable laptop (these Ultrabook toys would clearly have a much less capable GPU.)
That said, I wouldn't give up my desktop - I've had laptops, netbooks, PDAs (remember those?), and smart phones, and they all just make different amounts of compromise to give me a computer that I can move around a bit. My desktop, by comparison, doesn't compromise much (only in price, and maybe energy consumption,) and yields a much more enjoyable operating experience.
Of course, I tend to use computers a bit differently than many people ("You can't possibly need that many open tabs / windows / workspaces / monitors / etc!") and since computers are now used for 2 purposes (streaming video and facebook,) most people probably don't need more than a laptop. In fact, many of them probably don't even need all the buttons on the tablet screen keyboard - a simplified version with a handful of buttons for 'phrases' like 'lol' and 'omfg' would satisfy most of their text input requirements. For all the rest they can just click and drag the pretty pictures.