Epi - They're definitely a Christian act. The thing is - they also happen to write very intelligent lyrics, which may be why the Christian element isn't glaringly obvious.
I'm non-Christian & fairly liberal, but it's hard not to appreciate the clever wit of some of their lyrics -- take "Hey Mr", for instance, which ruthlessly satirizes Bill Clinton with extreme precision.
Most (or all) bandmembers are christians so it would be logical that their lyrics are (as galacticredskin said) in some way influenced by their faith. The lyrics don't have to be preachy to be christian.