Here are a couple of articles about the EyeFi card from a security researcher's point of view. They're very interesting and do make a few good points about how the card functions and what a few of its limitations are. It definitely taught me a few things about how the card works.
They also make the point that the EyeFi developers "get it" when it comes to security. That's very high praise, and I'm very glad to hear that they've been so responsive.
Thanks for the links, Ben!