I just spent a good portion of the day tryoubleshooting why we were getting this javascript error in out prod environment, but not in our development environment.  Specifically, the error was happening with a Telerik Popup control, but judging from a few google searches, I think that it is more specific to the ASP.Net Ajax extensions.

There a probably quite a few different causes of this problem, but I was able to fix it by removing the <xhtmlConformance mode="Legacy"/> node in my web.config file.  The xhtmlConformance setting appears to determine how controls render to the clients browser. The default is transitional.  It does not seem to play well with the Asp.net Ajax extensions if you have it set to legacy.

 UPDATE:
Looks like Scott Guthrie has already posted a gotcha on this.  Amazing that a google search for the error wouldn't pull it up.