The techies from Raspberry Pi have just announced that the ability to boot from a mass storage device with Raspbian installed without an SD card is one step closer today. There is now a public beta programme for the facility and you can get involved by using your own hard drive or pen drive. Head over to the Raspberry Pi Forums where you can find the instructions. Please also use the forum thread to feedback any issues with the operating system giving as much detail of your set-up as you can.
I’m part of the Alpha testing team and I can confirm that “it worked for me!” 🙂