This poll is pretty startling, but it does jive with the momentum Republicans seem to have in the last few days. From Santorum blog:

McCulloch polled over the weekend.

U.S. Senator Rick Santorum has gained ground in his bid to win re-election, according to a new poll released Monday. The poll of 800 very likely general election voters, was conducted by McCulloch Research & Polling, an independent polling company out of Chicago, on November 4-5.

Santorum’s opponent, State Treasurer Bob Casey, Jr., led the incumbent Senator 48.1-44.1%, according to the poll. “Senator Santorum has a history of closing hard at the end of his campaigns,” said pollster Rod McCulloch. “It looks like this campaign will be no different.”

Treasurer Casey racks up his largest margin geographically in Southeast Pennsylvania, where he leads Senator Santorum 53.7%-37.1% in the region encompassing Philadelphia and surrounding counties.

Senator Santorum, who racked up overwhelming majorities during his last election in 2000 in Central and Northwestern Pennsylvania, is seeing some of those voters returning to the fold, according to the poll. The second term Senator leads in Central Pennsylvania (55.1-38.8%), South Central Pennsylvania (55.0-44.1%), Northeastern Pennsylvania (54.6%-38.1%), and Northwestern Pennsylvania (48.3%-41.4%).

The Santorum Blog has links to all the latest polls in this race as well.