One of the VS.NET 2005 issues Microsoft hasn't provided a fix for...
Close the form, eventually build, then load the form in HTML, switch to design view and it should work. If it doesn't, close all forms, close solution, close vs.net, restart vs.net load solution, BUILD in full, load form in html, switch to designer.
It's so utterly strange why this happens, a lot of the datasource control writers run into this same problem. Lhotka of CSLA.NET found out that it seems to relate t temp assemblies vs.net creates behind the scenes... Though it's very odd, we had two dev systems, both had the same solution open, one could open the designer of a datasourcecontrol while the other kept the error, no matter what.