I came across an issue the the other day when adding a web part property to a SharePoint 2010 web part. My web part property was a List<> of a custom class and I was updating this property using a custom editor part.
I packaged and deployed the web part but when I hit Apply or OK in the toolpane I received the following error message:
Well it turns out I had forgotten to mark my custom class as Serializable! As soon as I did that the web part worked flawlessly.
Hope this helps