DotCloud brings one of the most interesting and missing features to the Cloud: High Availability for MySQL. Basically there are two MySQL servers that are configured as Master/Slave. The cloud Controller monitors the master server. If the master server goes down ( for any reason ) the cloud controlloer will switch to the slave, which becomes the new master. When the old master comes back up, it becomes the new slave, and the data is replicated from the new master.
What impresses me the most, is how it works and how easy it is to set up. Best thing about it, is that you don't have to change anything in your application to use it. I would love to see this feature implemented in other PaaS Clouds as well.
For more details see: MySQL HA in the Cloud