Numbers like these are the sort of thing that may nudge Castle into actually making a race of it, but bear in mind that Susquehanna is the same Republican firm that put out a somewhat dodgy-looking PA-Sen poll yesterday, so perhaps a grain of salt is merited here. A poll conducted in March by the much more reputable Public Policy Polling also had Castle leading Biden, Jr., but only by 44-36.
UPDATE: It also just occurs to me that Susquehanna is the same firm that released a poll last fall showing Democratic Rep. John Murtha leading his no-name Republican challenger by only five points. Murtha went on to win by 16 points in November.