I don't know if I should trust my non-native ears, but I've heard a couple of people (Katie from CollegeHumor is the first one come to mind) who say "thank you" with a voiced "th" instead of the proper voiceless one. Also quite a couple of people seem to have said "princible" instead of "principle" (like those guys from Corridor I think). I don't remember the exact videos where they say those words for demonstration, though.
So do Americans often voice their voiceless consonant if at all? Discounting intervocalic voicing which is more of a historical thing that happened centuries ago ("knives, clothes, Stephen"), this seems counter-intuitive to me, because it's more natural to devoice voiced consants, especially at utterance boundaries or if that consonant is next to a voiceless one (e.g., "slide" + a pause, in which /l/ is slightly devoiced because of /s/ and /d/ slightly so because of the following pause). Also discounting the North American tapped /t/.