yes they are, its very clear to me they take your card details and charge you after the trial runs out
In my case (and I suspect many others) they already had my card details because I use it to buy stuff from them. I don't agree that "very clear" is the same as "buried in the small-print" - still have the email they sent me:
Hello xxx,
Your Amazon Prime membership 30-day free trial has begun. Start shopping with unlimited One-Day Delivery on millions of eligible items, watch over movies and TV episodes with Prime Instant Video, and borrow one Kindle Book for free every month, with no due dates.
Happy shopping, watching and reading.
<<< Lots of pictures of cool films and TV programmes>>>
<<< small print in grey on white text which they know no-one will read>>>