Boot into Recovery by holding down the key combination command-R at the startup chime. Release the keys when you see a gray screen with a spinning dial.
Note: You need an always-on Ethernet or Wi-Fi connection to the Internet to use Recovery. It won’t work with USB or PPPoE modems, or with networks that require any kind of authentication other than a WPA or WPA2 Personal password.
When the Mac OS X Utilities screen appears, follow the prompts to reinstall the Mac OS. You don't need to erase the boot volume, and you won't need your backup unless something goes wrong. If your Mac didn’t ship with Lion, you’ll need the Apple ID and password you used to upgrade, so make a note of those before you begin.
Unfortunately this is actually not solving it for me.
I ran into the same issues as described by Karan. After a failed update with the latest stuff from yesterday, the computer doesn't boot may Mac OS partition properly anymore. I re-installed Mac OS ( Lion ), tried to update everything ( again ) and run into the same problems.
Did you install the latest Thunderbolt Update from Apple? If so then this is a problem with that driver. See here: http://reviews.cnet.com/8301-13727_7-57451440-263/thunderbolt-update-plaguing-ma cbook-pros-with-kernel-panics/?tag=txt;title