Now that we have serializer support for JSON and XML. The last one it makes sense to implement is YAML. Then we have the most common serialization formats out of the box. Plus, as we already have the YAML component, this is a relatively small amount of code.
This could open out some (possibly crazy) possibilities using rest web services with configuration too.
Here is an initial patch, we have to create the encoder from scratch as symfony doesn't have one, but this could be patched upstream later if possible?