This "Joe the Plumber", whose last name is Wurzelbacher, is (sort of)registered to vote. The problem is that his last name is misspelled in the registration database. The sleazy HAVA laws don't allow anyone to vote unconditionally if there's any letter (or digit in one's SS#) where the information on the registration doesn't match the information in other government databases. He might be forced to fill out a provisional ballot.
The purpose of this law (voted in when the GOP had a legislative majority) is to supress votes and is intended to primarily impact hispanic voters, but it will ensnare some non-hispanics as well.
Once a voter is forced to vote "provisionally", that ballot may or may not get counted. It falls to the discretion of the office of the Secretary of State.
I expect the HAVA laws to be revisited sometime next year.