Odin,
I was having similar problems with events not firing. There could be a couple things going on here, but I got it solved by doing the following:
1) Run the command: aspnet_regiis -c
This should take the new javascript libraries from the .NET framework and put them in your C:\inetpub\wwwroot\ directory.
2) Restart IIS (at least I did)
3) Make sure all your web applications are using the new aspnet_client directories. I was running into an issue where an application was using the old JavaScript libraries because they had been copied into an application directory.
As a note, I am running IIS 6.0, XP with SP2.
Good luck! This was a pretty frustrating problem; hopefully this will save some folks a lot of pain.
Dan