According to Rasmussen:

With former Tennessee Senator Fred Thompson taking his first formal steps towards a Presidential run and the immigration debate creating challenges for Arizona Senator John McCain, the race for the Republican Presidential nomination has an entirely different look this week.
Former New York City Mayor Rudy Giuliani (R) remains on top, but his support has slipped to 23%. That’s down two points from a week ago and is his lowest level of support all year. Earlier, Giuliani had consistently enjoyed support in the mid-30s. That was before Thompson’s name was added to the mix and before Giuliani stumbled on the abortion issue in the first GOP debate of the season.
Thompson, who just formed an exploratory committee and is the newest face in the race, immediately moved into second place. With 17% support, he is within six points of the frontrunner. That’s closer than anybody has been to Giuliani in 20 consecutive weekly polls. Thompson is also competitive in a variety of general election match-ups with potential Democratic nominees.
Among men, Thompson earns 21% support while Giuliani attracts 20%.
Former Massachusetts Governor Mitt Romney is the top choice for 15% of those likely to vote in a GOP primary. That’s little changed from the last couple of weeks and keeps him a single-point ahead of Arizona Senator John McCain who is preferred by 14%.

Has he peaked before he’s even formerly entered the race? We’ll see.
UPDATE: Pew also polled and says “Thompson has broad potential appeal among Republican voters.”