WebUIValidation.js / Framework Version Problem Solution
I just had one of my clients deploying some existing ASP.NET web applications to a few new servers. In the process, some of the client-side validation stopped working with the symptom of no post-back to the server when validation should have passed.
Running "aspnet_regiis -c" installed the correct version of WebUIValidation.js (and other scripts) on the new servers, and all is well.
I hope this might be of help to someone chasing a similar problem.