If you listened to Santorum’s speech Tuesday night/Wednesday morning, you’d have heard his family’s immigration history. His grandfather came to the US and became a citizen in five years, as was the standard at the time. He saved his money and brought his son (Sen. Santorum’s father) here as a young child @ 10 years old.
If his grandfather saw to getting US citizenship for himself in 5 years, one would assume he’d have done the same for his son. I don’t think we understand today how very important obtaining US citizenship was to immigrants during the first half of the 20th Century.
As to your underlying NBC question, his father was a US citizen long before Sen. Santorum was born, so he fits the Constitutional eligibility requirement under any interpretation.
Numerous sources say that it was his father that was an immigrant and none of them define when they were naturalized.
As I have said, my post was not about getting Santorum, it was about punching South40 in the nose for some of his BAD behavior.