Click on any of the "Run It" buttons for the samples:

http://asp.net/ajax/documentation/live/tutorials/IntroductionUpdatePanel.aspx
 

Yup, sure seems to be a problem. The javascript error is 'Sys' is undefined.

It's been broken for a couple months now... let's recap my initial experiences with ASP.NET AJAX...

(1) samples/documentation broken

(2) refreshes break the pages in firefox. (fix this by turning off view state and event validation)

That's not a very good way to introduce new users to the framework.  I think maybe Microsoft is trying to pass the buck to firefox on that second one, but don't you think they should make it work by default on the browser with 25% market share?  I lost three or four hours on that second one.
 

Been broken for months?  They are running fine with me on my WindowsXP box with Firefox.

What is your browsing environment? 

Windows Server 2003 x64 SP2, Broken in both IE and firefox (32-bit).  Did you click on a "Run It" button from the page i linked to?  The docs are there, just the samples fail. 

Like daytrip00 said, the problem is not with the documentation, the problem is in examples of the code:
http://asp.net/ajax/documentation/live/Samples/UpdatePanelTutorialIntro1/cs/Default.aspx
http://asp.net/ajax/documentation/live/Samples/UpdatePanelTutorialIntro2/cs/Default.aspx
http://asp.net/ajax/documentation/live/Samples/UpdatePanelTutorialIntro3/cs/Default.aspx

I checked each of those examples and they all appear to work fine. Can you provide an error message and/or screenshot?

bullpit:

Yup, sure seems to be a problem. The javascript error is 'Sys' is undefined.

I get the same error in IE6 and Firefox. 

dhounshell:

I checked each of those examples and they all appear to work fine. Can you provide an error message and/or screenshot?

I'm seeing similar results. The error I am getting is:

Line: 47
Error: 'Sys' is undefined

And my environment is IE7, Windows XP SP3. Same results for Firefox 2.0.0.14.