I'm surprised to see that the thread "MultiSelection FilterTemplate" Thread
on "http://www.telerik.com/server-error.aspx?aspxerrorpath=/community/forums/preview-thread/silverlight/data-filter/multiselection-filtertemplate.aspx" does no longer exist. I wanted to answer to RoadWarrior and surprisingly I've seen the thread is deleted...
P.D: To RoadWarrior. A new MultiiOptionFilter's version is on code library.