While I did think about that use for several days, I decided to get an iMac instead.
Here are a few of the reasons:
1. A refurb iMac was about $425 more than a ACD (Apple Cinema Display). For the extra money, you get a lot hard drive space and a really fast computer.
2. Unknown future capability of thunderbolt. If the thunderbolt cable goes the way of firewire on the iPod, it could render the display useless for future use.
3. A full computer has better future capability than just a display. The reason is that I can use Slingbox, Netflix, Hulu, iTunes, MOG, Pandora, etc.. with it. If I decide to update the display, I can use the iMac in the guest room as a way to watch video or listen to audio.
5. While I do see the benefit of having everything on one computer, I also see that it will be a pain to have to carry the laptop around and hook it up every time I want to use it with an external display. I mainly want to have the laptop available in the living room and do not want to have to track it down in my office every time I want to use it.
6. If one of the computers needs repair, I am still in business.
Also, I have a shared network drive that makes it easy to use files on both computers. Dropbox makes it easy to send files back and forth when I am away from home with the laptop.