EDIT: Disregard. I just re-built the solution and it cleared it up.
I just installed the 2010 SP1 release for VS 2010, loaded up a project, and updated everything. To make sure all was well, I grabbed a RadGridView from the newly created toolbox section and dragged it over to the design surface.
What I noticed different this time is that instead of adding this:
it added this:
As a result of this, when I try to add columns to the GridView, like this:
I get the error in the title of this message:
The property 'telerik:RadGridView.Columns' is not declared as an attachable property.
I am still able to use the RadGridView fine using the normal xmlns with the assembly reference - just wanted to let you guys know that this is happening on a fresh install, so others might experience the same.