I'm not sure if this will help in your situation or not, but there is an example of a dependent autocompleter in the struts-showcasee-2.0.x.war file that comes with the download.
One
word of caution, though: while this example worked fine for me using the FireFox browser, it did not work using Internet Explorer Version 7.
[ December 18, 2007: Message edited by: Merrill Higginson ]