While reinstalling Ubuntu is probably the easiest way, it's worth pointing out that it's possible to recover without reinstalling everything.

To do this, manually download the python2.7 package (and its dependencies), and manually install them using dpkg (bypassing APT, which requires Python). Once that's installed, apt should work again, and so apt-get install ubuntu-desktop will restore your system. (If apt-get still doesn't work, you might also need to download and install any missing dependencies.)

In the future, if you need to reinstall a package, instead use

sudo apt-get install --reinstall python2.7

or

sudo aptitude reinstall python2.7

which reinstalls the package without having to remove the dependencies.