fivethirtyeight.com has a fairly good analysis of this.

Byrd died at a fairly awkward time, election-wise.

(1) Had he died any time after July 2, the governor could have appointed someone to fill the vacancy through the end of Byrd's current term.

(2) Since he died a few days too early, the governor can appoint a temporary replacement, but a special election must be held.

(3) It is a bit unclear on when, precisely, this special election must occur. It appears too late to do it this year, since WV has already had their primary. There is some argument that the governor could put it off until 2012.

Regardless of when they decide to hold a special election, the seat won't stay vacant. The governor (a democrat) will appoint a temporary replacement to fill Byrd's seat until a special election.

Cloture, cloture cloture. Man I hope Harry Reid loses in November and the Dems wind up with 59 seats just so the new majority leader will be forced to grow some balls and change the rules on cloture to not be so idiotic.

One of the good guys...