In the above article, a key reference was wrongly replaced in the corrections. This reference should appear as [21a]:
A. L. Vázquez de Parga, F. Calleja, B. Borca, M. C. G. Passeggi, Jr., J. J. Hinarejos, F. Guinea, R. Miranda, Phys. Rev. Lett.2008, 100, 056807.
This reference has been reinstated in the print version in corrections made on June 24th 2009. The existing references 21a and 21b become 21b and 21c, respectively. The authors apologize for any misunderstandings caused.