If you'd rather do it on the server-side, I'd suggest "hitting" the service with URL.getConnection() in your servlet and parsing the returned response. HttpUnit is also a great tool for this.
What you probably don't want to do -- simply because it's a lot more work -- is to hit the service directly from the browser and try to intercept the response using filters and response wrappers. But that is a possible way of doing it. [ May 11, 2006: Message edited by: Bear Bibeault ]
P.S. <shameless-plug>If you are interested in the Ajax solution and you're a patient man, the upcoming Manning book "Ajax in Practice" will have a whole chapter devoted to using open APIs (google, Yahoo!, Flickr, et al) from Ajax.</shameless-plug>
Yes, as I mentioned in my first reply, filters are a possibility for doing this on the server side, but I think it would be a lot easier to pursue the URL connection route. But either means should work for you.