Shannon Hale actually answers the question of Austenland's steaminess rating on her blog. "Is this book appropriate for kids?" her readers ask, because Hale has written more than her fair share of young-adult books. Hale says, "If this book were a movie, I'd rate it PG." Fair enough. There's a bit of smooching, but none of the characters make it past second base.
However, we're going to bump it up to PG-13, just because we're totally skeeved out by Sir John's attempted sexual assault on Jane, and the fact that no one else seems to be as disturbed as we are by it. What gives?! We can only imagine what Jane Austen would think.