You can see how the editing of the header of the tree view is done in our examples project that is downloaded with the controls. Basically the text that is displayed in the editable textbox is what RadTreeViewItem.ToString() method is returning, which by default is equal to what RadTreeViewItem.Header.ToString() is returning.
An improvement to this mechanism could be when we start supporting the DisplayMemberPath property.
About your second question - the multicolumn treeview. I'd love to hear your feedback about how important this is for you. We have created an example about how to create a multiple column treeview for WPF, but unfortunately we are missing the GridRowPresenter control from WPF in Silverlight. The problem in Silverlight is how to share the columns sizes between all the nodes, because not all nodes will have the same size for the columns. Currently you can easily create a fixed sized multicolumn treeview, but I'm not sure how useable this would be.
Let me know your thoughts on this.
the Telerik team
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