Per the National Journal, here’s how it works:
The governor appoints a successor, but because there is more than 2 1/2 years remaining on the term, there must be a special election. The governor’s appointment is only valid until the special election.
But WV law requires that any candidates for the special election be chosen in a primary election. WV’s primary occurred in May. The next primary election is in 2012. Thus, in November 2012, there will be two elections: a general election for the six year term, and a special election to fill the last few weeks of Byrd’s term.
http://hotlineoncall.nationaljournal.com/archives/2010/06/filling_robert.php