We attempt to provide 100% coverage of functionality with the Web Services API - but, it tends to lag behind the foundation services.
Moving forward, we'll continue to invest in the Web Services API (which can provide client-side java bindings). However, we're also starting to invest in a RESTful Web Services API, which we've started by introducing the Web Scripts framework in 2.1. Over time, we'll provide a formal supported HTTP API, likely to be based on specs such as ATOM/APP etc. Client-side java bindings will be provided too, however, 3rd party java libraries such as ROME etc will be usable against such an open api.