Article on it:
Most discoveries of non-citizens on the registration rolls are therefore accidental. Though the Department of Justice has no procedures in place for a systematic investigation of these types of criminal violations, in just a three year period, it prosecuted and convicted more than a dozen non-citizens who registered and voted in federal elections in Alaska, Florida, the District of Columbia, and Colorado. [32] Among them was an alien in southern Florida, Rafael Velasquez, who not only voted, but even ran for the state legislature.[33] Eight of the 19 September 11 hijackers were registered to vote in either Virginia or Florida-registrations that were probably obtained when they applied for driver’s licenses.[34]
http://www.heritage.org/research/reports/2008/07/the-threat-of-non-citizen-voting
Not to mention the Cubanadian® Senator who ran for President.