Many customers have asked for a control to select multiple values for a field, or to tag work item with various tag strings & to be able to query on them. We might have such control built-in in future releases, but until then this can be solved with a custom control. We built such a custom control and posted it in codeplex project:  http://www.codeplex.com/WitCustomControls

This uses a UI only solution for multivalue problem. We do not have any "multivalue" fields in our OM. See here for details on how it is implemented with just string field & how to query on its data: http://www.codeplex.com/WitCustomControls/Wiki/View.aspx?title=Multivalue%20control&referringTitle=Home