As you may have gathered from the news, McCain picked a hardcore evangelical for VP and Obama picked a hardcore Catholic.
If this matters state by state, this is how it will likely do so in crucial states:
An evangelical could help in: Virginia (32.7% evangelicals to 14% Catholics as a percentage of overall Christians), North Carolina (40% e, 11c), Georgia which may end up close (32.7e, 8.9c), OR, SC and AK which lean McCain.
Missouri has plenty of both (27e, 19c) but where it's close keep in mind that Catholics are more likely to be undecided while evangelicals are mostly counted already in the Republican base, but with potentially increased turnout and mobilization. We can also presume that evangelicals are more inclined to vote for their own than Catholics.
Florida's Latinos put Catholics slightly over the top (26c, 24.6e). Ohio is 25e 19c, Colorado 25e 23c about 10% Mexican, Michigan 25e, 23c.
A Catholic could help in: New Mexico (40.2c, 30% Mexican, 18e), New Hampshire (35c, 22.3e), North Dakota (30c, 16.2e), Pennsylvania (27.4c, 21.5e), and Nevada (23c, 20e). The 5.9% Mormons in Nevada like others there may resent Palin calling out their senior Senator Harry Reid.
What percentage of evangelicals will say "Sarah Palin is a cheap attempt to get me to vote for a dangerously unqualified potential president"? We'll see...
This analysis suggests that if Palin conducts herself perfectly and communicates a command of the issues, she could take a few votes from Obama in Virginia and force him to win it in other states. This is a minor effect on a race that represents the best case scenario for the Alaska governor. Obama's position looks very strong even if this happens.