We assume that all the candidates seeking this race during the regular election (March 11 Primary; November 4 General) will qualify for the special election, but we expect the Republicans and Democrats who do not win their party's nomination will drop out of the special after the primary. But maybe not, it could get interesting.
Clarion Ledger - Special election set for Wicker's old seat - A special election to fill the unexpired term of former U.S. Rep. and current U.S. Sen. Roger Wicker in Mississippi's First Congressional District has been set for April 22, according to a news release from Gov. Haley Barbour's office. In a Writ of Election filed with the Mississippi Secretary of State, Barbour said if a runoff is required, it would be held on May 13, 2008. He designated March 7, 2008, as the qualifying deadline for candidates in the special U.S. House election.
Commercial Dispatch - Barbour sets special election timetable
CQpolitics - Multiple Mississippi Election Dates Hard to Keep Straight