When you use VS 2008 to open an existing ASP.NET 2.0
application that uses ASP.NET AJAX 1.0, you can optionally choose to upgrade
the application to use .NET 3.5 (and the version of ASP.NET AJAX included
within it).
The VS Web Tools team recently published a Upgrading ASP.NET AJAX 1.0 Websites and Web Applications to
.NET Framework 3.5 blog post that describes the step-by-step
instructions to accomplish this using VS 2008 Beta2. The good news
is that upgrading an ASP.NET AJAX 1.0 application to .NET 3.5 does not
require that you change any of your code, and should take only a few minutes to
complete.
As part of upgrading ASP.NET AJAX 1.0 application to .NET
3.5, you'll want to update compiled ASP.NET AJAX control libraries you might be
using. The ASP.NET AJAX Control Toolkit team is now publishing both
ASP.NET AJAX 1.0 and .NET 3.5 versions of the AJAX Control Toolkit that you can
download here:
Figure 2
The Upgrading ASP.NET AJAX 1.0 Websites and Web Applications to
.NET Framework 3.5 blog post covers how to add this ASP.NET AJAX 3.5
version of the AJAX Control Toolkit to the VS 2008 toolbox.