Hi, Joel. The MLO people have never promised to encrypt data-at-rest on the cloud server, so you have to assume that it's stored in the clear.
Since you asked about access to private keys, I wanted to note that the sync process does not require the server to "see" the content of any of your tasks, or even to be able to compare two copies of a task and detect if there has been a change. As long as every task has a unique ID that's the same across all platforms all the server needs are the ID and some timestamps. That means that the record could be encrypted on your device and not decrypted until it arrives at your other device. There would never be a need to question whether there had been a security breach or privacy violation at the server. Any slowness from crypto overhead would be on the user's device and would probably appear as battery drain, easily cured by turning off the crypto.
But that's just a discussion. In the meantime if you want to know that your sensitive information is secure (beyond the password-grade security for at-rest data and SSL for in-flight data) you should move the sensitive data out of the task and store it securely in a file at a location accessible to all of your devices, and then include in the task a link to the file.
-Dwight
Mlo betazoid on Android sgn2