In ECF's Helios release, we released an implementation of the OSGi remote services standard specification (chapter 13 in compendium).

In addition to the full spec implementation...which is based upon synchronous remote service proxies...we added support for asynchronous remote services. This provides non-blocking access to remote OSGi services. This gives remote service consumers choices...allowing them to invoke remote services synchronously (i.e. by making a blocking method call on the proxy), and/or asynchronously (with a guarantee that the calling thread will not block).

I think that one nice thing about this approach is that the service host implementer has to do exactly nothing to make these consumer choices available. The implementation of the service host is exactly the same.

There are two styles of asynchronous access supported: an asynchronous callback (like GWT), and a future result, from the Actor model of computation. These two styles of of asynchronous access...along with the specified synchronous proxy...provides remote services consumers with some useful choices for creating reliable distributed systems and applications.

From http://eclipseecf.blogspot.com/2010/09/asynchronous-remote-services-choices.html