Hm, are you sure it is of type System.Guid ?
I just modified standard demo to contain additional Guid column bound to treeview:
dTable.Columns.Add(\"Value\", typeof(Guid));
then
dTable.Rows.Add(new object[] { 2, 1, \"Spices.Net\", true, Guid.NewGuid() });
All is ok.
The error you get is thrown from the follwoing code:
Convert.ChangeType(value, type);
value is your value (Guid as you say) and type is type of node.Value property (System.String).
Could you please get exact yourObject[\"ID\"].GetType().FullName?
Thanks in advance.