Since transformer are often concerned with enrichment/filtering of particular values in the MessageHeader, we also should consider more fine-grained annotations (including parameter-level) for binding from the Message to method arguments. This will be consistent with the annotations used in Spring 2.5+ MVC.
For example, something along these lines (just the general idea)... setting a value from a header:
public void addEmail(@HeaderAttribute("email") String email, Customer customer)
The same basic idea can be applied for providing a value to a header - or possibly returning a Map of values to add to the header (similar to a "referenceData" returning method in Spring MVC). Obviously this example is so trivial that an expression-language based solution would be sufficient (either in XML or class-level @Transformer annotation).
These same annotations will be useful for routers.