From AP:

Romney, however, is employing a novel fundraising strategy. He established not only a federal PAC, but also affiliated PACs in five states: Iowa, New Hampshire, Michigan, South Carolina and Arizona, although the one in Arizona is now closed.
In so doing, Romney has been able to accept the $5,000 maximum per person for his federal PAC, as well as additional money from the same contributors for his various state PACs. Federal officeholders exploring presidential runs, such as McCain, Sen. George Allen, R-Va., and Senate Majority Leader Bill Frist, R-Tenn, are prohibited from operating similar state branches.