YAML is a very nice format for configurations and is very popular in the Symfony world. However, it is not supported by Eclipse IDE natively (Eclipse simply sees it as a pure text file with no highlighting at all).
Luckily, there is a plugin you can install in Eclipse to take the advantage of this powerful file format.
Go to http://code.google.com/p/yedit/ and follow the installation instructions or simply add http://dadacoalition.org/yedit into your Eclipse install sites.
Happy YAMLing..