The XamOutlookBar OutlookBarGroup has a memory leak related to the datatemplates assigned to LargeIconDataTemplate and SmallIconDataTemplate. This is not the same issue as with the intrinsic Silverlight DataTemplate memory leak that was supposed to be fixed in the last SL4 update. This leak occurs whether you put the DateTemplate into the page resources and reference it from the control or not. Either way it holds a reference to the page or user control and won't allow garbage collection. I fixed it in my app by manually setting LargeIconDataTemplate and SmallIconDataTemplate to null before exiting the control or page.