Well, the DB columns will be hard to change, and everything kind of follows from there.  The Data API isn't strictly a Python API - and most REST interfaces I've seen use underscores.  We're always going to have a mix of capitalization styles, just by virtue of being a Python app (that pesky stdlib and its pep8!).

I think you could argue either way for the data API.  And so far, it's underscored.