Mind your Versioning Strategy
Arjen quoted Haralampos Routis’s reply over at the Spring Web Services Forum stating: The implementation of the interface is important but it can also change in time. The interface on the other hand cannot/should not change. Thus invest on XML Schema and WSDL, because these are the essence of Web services.
To me an interface implies a couple of things, namely, a service contract and data/message contract and their implementation details, with them, not necessarily having the same versioning strategy.
Service contracts, data/message contracts are amended over time, this is the reality. One strategy for a service contract is to add a new contract but continue to support the original contract. Data/message contracts however have a build-in versioning system, assuming you use XML to describe the message body. I think my point is, yes I wholeheartedly agree that you need to invest in the service contract (WSDL) and data/message contract (XML), but I also want to stress the importance of the versioning strategy.