The New York Times’s take:

His decision to drop out could potentially help Mike Huckabee, the former governor of Arkansas, who has competed for many of the same voters. Some Republicans suggested that Mr. Huckabee came in second in South Carolina precisely because Mr. Thompson had siphoned off much of his support, permitting Senator John McCain of Arizona to win.
But Mr. Huckabee has moved to scale back his own campaign after his South Carolina showing, and has backed away from plans to campaign heavily in Florida.
Assuming Mr. Huckabee does not reconsider, Mr. Thompson’s withdrawal could therefore be a boon for Mitt Romney of Massachusetts, who has aggressively sought to recruit Florida conservatives to his side.