The servicemix-http component does not support http digest authentication. Looking at http://issues.apache.org/jira/browse/CXF-2100 it seems that this feature was recently added to CXF, so servicemix-cxf might have it already.
Also, you might want to consider using SSL if you require the password to be protected.
Support for HTTP digest authentication was added in one of the latest versions of CXF (so it might not yet be available in servicemix-cxf but will be soon).
In your Spring http:conduit config, you should be able to set the "authSupplier" to a spring bean of type org.apache.cxf.transport.http.DigestAuthSupplier. That should automatically configure the rest of it.
Haven't tried it myself though.