Team System Web Access 2008 powertool is released with TFS 2008 recently on Nov 19th (see here for WIT improvements in this release). So I expect more people will start looking into porting their custom controls over to Web Access or building new ones in Web Access. Few asked me how to building custom controls for web access, and here is how from Hakan who owns TSWA:

Our SDK is shipped along with Team System Web Access itself, so you will have it when you install TSWA.

You can download our CTP release at:

http://www.microsoft.com/downloads/details.aspx?FamilyID=8d3f8eec-301a-4e96-adc5-abf47f462654&DisplayLang=en

Once you install it, look under the installation directory – you’ll see an “SDK” directory with the documentation and samples for WIT custom controls.

Custom controls built for VS winforms cannot be used as-is in web access because of technology differences, but it is as easy to build new ones for Web Access. Check out the SDK.