The Mac OSX 10.6.6 update Apple released yesterday to support the new Mac app store, may kill your mac... or at least your hard drives file system.
Apparently this has happened to at least a few hundred folks... probably a lot more... given how many people are popping up in forums.
What's happening, is your catalog is being corrupted somehow, during the install and reboot process; and it can't be recovered.
So far, no particular common elements have been identified to isolate what's causing it in the relatively small number of users who have had the problem; except that it seems primarily to be happening to people who have vmware, virtualbox, or parallels installed... but that's at least 1/2 of all of Mac users these days, and there's no clear indication if it's causative or not.
Also it seems to be happening more to people with SSD's but not exclusively.
If you've installed the patch already, don't reboot. Make a carbon copy of your drive, just in case; THEN reboot. If your drive is hosed, you can fix it.
And yes, of course, you should have done a backup before you updated... Yeah... how many actually do that for their laptops?
If it's already too late for you, there is a solution. Grab an external drive, and a boot disk (they can be one and the same if you've got two partitions on the drive and dont mind killing one of them), and you should be able to mount the trashed drive read only, and then carbon copy it to a partition on the external drive.
Best of luck.