That is the result of the Public Policy Polling poll that they completed this week (as we all voted on last week). From the PPP blog:
Two major trends have shown up in both polls to give Castle the early advantage. The first is an overwhelming 52-23 lead with independents. That mirrors a national trend of strong Republican performance with that group, although Castle’s edge is particularly large. The second is that he wins over far more Democrats than Biden does Republicans. Castle has a 79-10 lead within his own party, while Biden is up just 65-20 in his. Castle is winning more support from people who approve of Obama than any other Republican Senate candidate we’ve polled on nationwide this year.
They still didn’t specifically poll O’Donnell, but:
Castle may get a bit of a challenge from Christine O’Donnell in the Republican primary because 30% of the party’s voters think that he’s too liberal. Compared to other Republican Senate candidates across the country Castle is unusually popular with Democrats, as 48% view him favorably, but also unusually unpopular with Republicans, just 61% of whom view him positively. It’s unusual for the gap in favorability for a politician between the two parties to be so small.
Poll has a MOE of 4.1% . Crosstabs (pdf) here.