I had a case on this a few months ago, but thought more folks might run into this as they start moving to form regions. By default, if you use the VSTO template for creating a form region in Outlook, the item is leaked. This can show up in a number of different symptoms, but the one we saw was that when the item was created (then saved and closed), then modified in WebDav, then they tried to modify the item again in Outlook, the user would get the error:
"The item cannot be saved because it was changed by another user or in a another window."
Fortunately, the resolution is easy, you just need to add a call to Marshal.ReleaseComObject to the FormRegionInitializing event in the designer code.
private void FormRegion1Factory_FormRegionInitializing(object sender, Microsoft.Office.Tools.Outlook.FormRegionInitializingEventArgs e)