Unfortunately, there is a confounding variable. Catholic populations are higher in urban areas than in rural areas. People in urban areas tend to support Romney, people in rural areas tend to support Huckabee. So we have a negative correlation between Catholicism and votes for Huckabee that can be explained by looking at a third factor. Another reason to find this alternative explanation more convincing is because Catholics mostly vote Democratic anyway, so only a few would be voting in the Republican primary.
Let me first say that my analysis is merely suggestive and was intended only to shed some light since the exit polls did not ask about religious denomination, only born-again status. Second, I'll be the first to admit that county-level analysis can be a pretty crude measure of things and there are obvious problems of ecological fallacy when you use any analysis with aggregate data.
But those things said, I did try to account for the confounding variable urban vs. rural residency. My regression included a variable for the percentage of population living in rural areas and even when you control for this factor, the variable for Catholics was still statistically significant. As for the possibility that few Catholics are Republicans, there isn't much evidence. The exit polls from 2004 show that Bush got 46 percent of the Catholic vote in Iowa.
Of course, this issue could be cleared up if we had some polling data that asked about religious denomination. A a recent Pew poll suggest that Huckabee does do poorly among Catholics, but his real problem is with mainline Protestants. Huckabee had the support of 28 percent of evangelical Protestants and 17 percent of Catholics, equal to his overall percentage. On the other hand, only 4 percent of mainline Protestants supported him.