Lion has a very interesting function to recover from errors and reinstall the operating system without the need for a disk called Internet Recovery, but unfortunately it only works on a few Macs that didn't include even the MacBook Pro (Early 2011).
Luckily for the owners of one of these, Apple has updated the firmware of this computer to include this featureAlthough it is not desirable to have to use it, it does help in emergencies.
To install this update you must first download it using this link.