First of all THANK YOU! You're the author of heaps of plugins I use, so thank you for all you've done so far :)
Sadly though, there seems to be an issue with the latest version of the Mad Mimi plugin which I have installed on two sites http://www.competitions.ie is one of them.
The plugin is not syncing registered users with my madmimi account, even though when setting the plugin up using api key etc, it authenticates and shows a list of my lists. I'm running the latest version of WordPress, and latest version of the plugin, and my server spec meets your requirements.
Any ideas (I fully understand this could be a Madmimi Api issue too!).