Yes RDS is like any MySQL db. The key word there being MySQL. If you want to use another data store like say, postgres or even some NoSQL solution. You're outta luck.

As for platform lock-in, my comments were less towards the web services level and more about system level concerns. It's not entirely clear how you would run persistent background tasks (read: celery). This doesn't even get into the headaches of learning yet another DSL to define OS packaging requirements.

That being said, IMO this would be great for simple apps, with no strenuous requirements. In addition, anything that allows more people to easily deploy their django apps is a good thing.