VS2010 Client Profile - Namespace Reference Invalid After Build

So this is the scenario, in a project, add a reference to an assembly, then declare a variable of a type from the newly referenced assembly. Then type in the declaration, and you get the usual blue squiggly line. If you right-click then you get the option to add a 'using' reference to my assembly, and then bobs-your-uncle, all is good...

Until, you try to build the project, then for some ungodly reason VS no longer regards the namepace inclusion as valid, and spits out a

Error 377
The type or namespace name 'xx' could not be found (are you missing a using directive or an assembly reference?)

error, referring to exactly the same assembly ref that it just suggested be included.

This is, apparently, a dotNET 4 Client Profile Issue.

http://www.go4answers.com/Example/lost-references-while-building-project-143531.aspx
http://msdn.microsoft.com/en-us/library/cc668079.aspx