I've added a demo to the DWR site on how to do server-side validation that appears to be run on the client.
It also demonstrates how little boiler-plate DWR requires. The Java code from the examples below is taken from Jakarta commons-validator, without any changes. We simply told DWR to export pre-existing Java functions that were never designed for the web, and yet it all works just fine.
Check it out here: http://directwebremoting.org/dwr/demo-validate.html.