Here's another entrant using the NOAA charts: Skipper | NOAA charts for iPhone and iPad
I'm not in the US but it doesn't seem like they've got the scaling for the various zoom levels quite right yet. Their pricing model seems to be charging for offline use for when you're out of cell phone range. I don't know how I'd feel about using it as a primary source but I can see using it to pre-plan a trip. They're building out their website too to add most of the functionality there.
I wrote a MAFOR decoder iPhone app
, mostly because I wanted it myself. I'd love any feedback.