What a great idea! If you move the operating system to flash memory your computer will boot up much faster. Another bonus is that flash memory is much more energy efficient than hard drives (no moving parts), so laptop batteries would last longer.
If you also move the swap file to flash memory you'll realize the same benefits -- increased speed and longer battery life.
The major obstacle to this is price. Flash memory is very expensive. One other obstacle is capacity. Flash is currently limited to about 8 GB.
See http://www.technologyreview.com/Infotech/18420/ for more details.