I've implemented this feature request. My contribution consists of two parts:
1) a patch for camel-core to add the spel() language construct to the DSL and Spring configuration. See: http://github.com/rkettelerij/camel-spring-expression/blob/master/camel-core.patch
2) a new language component called camel-spring-expression that adds Spring Expression Language support to Camel. See: http://github.com/rkettelerij/camel-spring-expression for code and docs. Test coverage is around 75%.
I've implemented the language support in a separate module (like camel-mvel and camel-ognl) to have camel-spring be compatible with Spring 2.5. This way we can include it in Camel 2.6. When work on Camel 3.0 starts is probably better to transfer this code to the camel-spring module.
The source code in the referenced GitHub repo is Apache 2.0 licensed. Let me know if it's up to your standards, then i'll put the documentation in Confluence.