I found this gem from Ben Alex on the Spring Forums tonight. fValidate looks to be an awesome client-side validation library. If you do a view-source on the aforementioned page, you'll see that all the validation rules are specified in the "alt" attribute of input elements. Apparently this is XHTML valid. Since the current implementation of WebWork in AppFuse doesn't have client-side validation, I may have to hack something together tonight. From the looks of it, this will simply involve adding an "alt" attribute to the existing WebWork tags.
Later: I think I'll pass on the AppFuse/WebWork integration - it looks like a fair amount of work and I'd rather just get 1.6 finished and released.