I don’t think you can recall a federal officeholder, that sounds like it would be unconstitutional. But an investigation should come forward and if it proves enough votes were suspect, Tester should be expelled by the Senate (for which they have grounds to do so) and Burns seated or a new election called. The question is whether Burns is game to file a complaint, assuming he still can.
You can’t recall a U.S. Senator, but the Senate can judge him not to have been elected and thus exclude him from the Senate, even after he is sworn in, if there’s an election challenge. While the Senate may have some internal rule that forecloses election challenges after a certain time has elapsed, certainly nothing in the Constitution would prevent the Senate from excluding Tester upon evidence of voter fraud.
However, I wouldn’t hold my breath here. We’ll have to wait until 2012 to win back Tester’s seat (unless he suffers a debilitating stroke, in which case we won’t even challenge his reelection).