As Brian said, you can access a web service from Struts in the same way you would access any application model -- via the Action. Instead of calling a method that returns a result from a database, you would call a method that returns a result for a web service.
Ideally,
you should try and encapsulate your business tier behind a facade. The Action doesn't need to know where the data comes from. It just knows it needs to call this method with these parameters. The Action can then return the result to the view (via the request context).
The Artimus application bundled with Struts in Action makes calls to both a database and the Lucene search engine. But all calls are behind a consistent facade so that the Action doesn't know where the result comes from. This makes it easy to add another persistance device, like a web service.
HTH, Ted.