I was reading an entry from Paul Wilson's blog this morning, Service Pack 1 for .NET v1.1 Broke My ASP.NET App, and realized that I am having the same problems. Scott Hanselman wrote about it a few days ago, too.
Yesterday I was demonstrating some updates that I had made to a project to our internal team when we realized that something was amiss. Pages with validation controls for form submittals are not behaving properly - they were attempting to validate the form after clicking an asp linkbutton control that is supposed to take the user to another page. After reading Paul's post I now realize that it probably was only client side validation in IE (we were all using IE at the time). I know that the click on the link didn't cause the validation before, but I thought that I might be going crazy. I also remember seeing something similar in an internal application that I was working on a few days ago, but brushed aside at the time as something I just had not fixed yet.
I think that the main reason that I haven't noticed the issue up to this point is that I primarily use Firefox now and Paul reports that the “bug” only happens with IE, not Firefox (since it is considered a down-level browser).
Hmmm, first off, I don't even know if I have any of those pre-SP1 files... I'm pretty sure I've updated every machine that I have any control over with the Service Pack. Secondly: I don't understand how installing the new validation files AND replacing the new ones with a copy of the old ones can both the valid solutions ?!?!
Well, I guess I am off to fiddle around with my servers.
UPDATE: The aspnet_regiis -c thing didn't seem to do the thing for me - well, technically it did work, but it just copied the new files into place. This is apparently not the action that I wanted, though. I was able to fix the problem that I was having; I just set the linkbuttons mentioned above to have CausesValidation=False. That did the trick, but I'm still a little ticked that the updates in the SP caused behavior of existing code to change.