That's great news, though I still can't get it to work...
Your link pretty much outlines the steps I've already performed. I added a new tab to my toolbox, right clicked on it and selected Choose Items. Then I browsed to Telerik.Web.UI.dll under my site's bin folder and checked all the Telerik controls. They all appear on the toolbox, so no problem there. However, when I add one of them to one of my user controls, I get the following error in design view:
Failed to create designer: 'Telerik.Web.UI.RadTextBox, Telerik.Web.UI, Version=2009.1.410.35, Culture=neutral, PublicKeyToken=121fae78165ba3d4