Hey fraXis,
Great question.
Our servers have a little more than 99.9% uptime. Have have daily secure offsite backups of our databases, and we have fail-over servers on the other side of the country in case of floods, earthquakes, or other natural disasters.
Plus, TurboActivate (the part that integrates with your app), has built in grace periods. That is, if the internet connection fails (on our end or on the customer's end) the user will be able to continue using your app and can try to activate again in a few hours. The grace periods are completely customizable (or you can disable them altogether).
Lastly, we offer LimeLM to run on your own servers. Some of our larger customers choose this option. It's easy to install and it works on all major server software (Windows, Linux, BSD, etc.).
Does this help?