I see that the Washington Post says Brown is looking better, and illegal immigration is one of the factors:

"The word I hear most is 'disgusted,' " said Fred DeFinis, 58, who has worked the phones for the past two weeks. Seven to eight of every 10 independents tell him that they will vote for Brown, he said, proportions similar to those seen in some public opinion surveys.

"It's very hard for them to say what's on their minds," said DeFinis, a registered Democrat. He reads from a form asking whether they are most concerned about: (1) jobs and the economy; (2) the health-care bill; (3) illegal immigrants; or (4) national security. " 'All of the above' is what we hear the most," he said. [Polls show race for 'Kennedy seat' about even By Karl Vick, Washington Post, January 15, 2010 ]

You'll notice that even though the Post quotes Scott Brown's brilliant line from the debate:

"The defining moment came when moderator David Gergen asked whether Brown was prepared, if he won "Teddy Kennedy's seat," to cast the vote that killed the health-care reform bill.

"With all due respect," Brown shot back, "it's not the Kennedys' seat. It's not the Democrats' seat. It's the people's seat."

the post headline still refers to it as the Kennedy seat.

See also GOP’s Scott Brown Could Win Teddy Kennedy’s Senate Seat–If He Plays Immigration Card and Matthew Richer's more pessimistic estimate here.