Apple recently started seeding OS X 10.7.3 through Software Update, and while a majority of users upgraded without a hitch, some are experiencing odd “CUI” issues and frequent crashes. If you’re part of this minority, head over to Apple’s support site and install the 10.7.3 client combo update over your broken system. If your system crashes before you can make it to the desktop, try booting into safe mode by holding the “shift” key on boot.
This should resolve the issues, but if you are still unable to boot to the desktop to install the combo update, you’ll need to either restore to a pre-10.7.3 Time Machine backup or reboot into Recovery mode and reinstall the OS X base system (if you do not wipe your hard drive or restore from Time Machine, your data should be left untouched). Hopefully, Apple will sort out the bug on their end before the problems become widespread.