Ooops. How can I re-install the Mac App Store?
Many options are available to you:


Restore it from your backup
Copy it from another Mac running 10.7.2
Download and apply the current combo updater (often they contain entire programs, but it's not a guaranteed thing)
Reinstall Lion and upgrade to 10.7.2 - if your Lion install is lower than 10.7.2 try not to run programs until they are all patched up to the same version as your existing software.