If your application uses encryption in any way, even SSL or HTTPS, you need an Encryption Registration (ERN) from the US Bureau of Industry (BIS). I recently got one for an app that I got in the Mac App Store and I documented my whole journey to share it and be a guide for other people needing to do the same. It's actually not very hard, as long as you know the steps to take (twice I needed to call US agencies to figure out the next step). Here it is: How to legally submit an app to Apple’s App Store when it uses encryption (or how to obtain an ERN).
Enjoy and feel free to ask any questions, I'll try to answer them and/or update the guide to the best of my knowledge. Oh... and disclaimer, I'm not a lawyer and this is not legal advice.