Now here's a map of the distribution of Catholics in Iowa (red equals the most Catholics):
This suggests that Huckabee did very poorly among Catholics. Unfortunately, the entrance poll of caucus-goers only asked about born-again or evangelical status, not denomination. A quick regression of the county-level data backs up what the maps suggest; Huckabee did best among evangelicals in rural areas with lots of religious adherents. On the other hand, he did poorly among Catholics. This indicates that he will have an increasingly tough time in states without a large evangelical population and particularly in states with large Catholic populations.
|% of Catholic congregations||-.67||.15||**|
|% of Evangelical congregations||.45||.12||**|
|% of population rural||.13||.03||**|
|% of population religious adherents||.002||.00||**|
Adj. R2 .46
** statistically significant (p<.01)