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.
As it turns out, the Framework was recently installed on the new servers with all of the latest patches; however, when the applications were moved from the older servers, they also moved the files from "aspnet_client/system_web/1_1_4322" to the new servers. This apparently created a conflict with the WebUIValidation.js file that was downloaded to the client, and the JavaScript code generated for each page sent to the client browser.
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.