Problem:

I just upgraded my TFS 2008 to 2010 and my custom TSWA controls are no longer saving values.

Solution:

Make sure the first thing you do in the InitializeControl() method from the IWebWorkItemControl inteface is call the code that populates the base.Controls collection.  For example to get the MultiValue control to work you must add a call to EnsureInnerControls() as the first line in InitializeControl() otherwise the values will not save.