Before you start building your next web application, powered by a hybrid data backend of MongoDB and MySQL (or any other relational database), you should consider some of the problems you might face.
Data consistency: If you use MongoDB as a cache-tier on top of a relational database, you will need to keep it consistent with the changes in the underlying data. You can run background processes that are fired at a certain time, and update the stale data in MongoDB. A more elegant solution would be to define callback methods in the data access layer, which will automatically update the MongoDB data every time you insert/update/delete something in the tables.
Complexity of the software architecture: From the application developer's point of view, having both MongoDB and an RDBMS as the data backends increases the complexity of the code. This is because he now has to provide and support two separate data access layers, one for the MongoDB database...