instead of just
"x-tree-node-icon" it needs a class that reflects the collapsed/expanded state
currently when using the cls attribute on a treenode to style the node icon it's impossible to nest folders with different expanded/collapsed icons and style them properly for IE.
this problem only occurs in IE6+7 when having folders nested and and different expanded/collapsed icons applied. and only on the expanded state!
a possible fix would be to apply the replaceClass() in Ext.tree.TreeNodeUI.prototype.updateExpandIcon not only to this.elNode but to this.iconNode as well.
see this thread for more details