So I was building my user control in SharePoint, no rocket sience I thought. When I opened my default.aspx page with the user control, the error 'could not load type ...' is shown. After some struggling and experimenting, I found out that my dll which was deployed in the GAC needed to be moved into the BIN folder.

Maybe there is a good explanation for this, but I have no clue ;) So my conclusion is to deploy the ASCX / ASPX code behind assemblies in the BIN folder of the web application.

Have fun!

Update 4 March 2010: Also when you get the error 'Could not load type ...' from a code behind file in for example a masterpage, try to move the assembly to the BIN folder