At a guess michael may have been using a beta or rc version when that tutorial was created.

Im currently working in 2008 with the sp1 and .net 3.5sp1 beta and 3.5.0.0 works perfectly for all my testing.
<add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

Westa

I'm fine with it not working on older sites since this is for our corporate intranet and I have full control over that.  What is weird is once I changed the reference to 3.5, I am getting a warning that 'asp:ScriptManager' is ambiguous.  Please modify the associated registration that is causing ambiguity and pick a new tag prefix. (this on an ASPX page in my module).

So I can change the TagPrefix and get it to work, but I don't really see why it won't work with the "asp" TagPrefix like the 2.0 version did.  Ideas?

*EDIT* -- apparently I had a few UserControls that were specifically targeting that version and I hadn't removed that reference.  I also added the assemblyBinding section under runtime at the bottom of my webconfig to hopefully forward all others that I may have forgotten.  Seems to do the trick.