It's still very much a work in progress. If there's enough interest and the app provides enough value to warrant it, I would eventually like to make it a subscription-based app. I'm perfectly happy to leave it free if as an example app in my portfolio if it doesn't take off.
Any and all feedback welcome, here's the blogpost with more info:
http://najafali.blogspot.com/2010/08/introducing-cardmonkey.html
Show HN: Another Spaced Repetition Flashcard App
cardmonkeyhq.com