Ben I've got this same problem too. I'm using Q1 and the Telerik.Web.UI namespace does not appear in the RadDock.Net2 assembly. Are you sure this is not located in another assembly or perhaps in a later or earlier version than Q1?
In fact a lot of stuff does not appear to be the same in Q1 2008. For instance I cannot seem to find RadDockLayout or DockState?
Nevermind please disregard my reply, as I was not using the latest version of telerik webcontrols. I need to use the prometheus version which has Telerik.web.ui.dll and I was using the older version (radnet2).