The IceUserObject has a toolTip property but the TreeRenderer doesn't do anything with it:

suggested fix:
Index: D:/Documents and Settings/pbreau/workspace/ICEfaces/component/src/com/icesoft/faces/component/tree/TreeRenderer.java
===================================================================
--- D:/Documents and Settings/pbreau/workspace/ICEfaces/component/src/com/icesoft/faces/component/tree/TreeRenderer.java (revision 14872)
+++ D:/Documents and Settings/pbreau/workspace/ICEfaces/component/src/com/icesoft/faces/component/tree/TreeRenderer.java (working copy)
@@ -384,6 +384,9 @@
 
         // efficiency and simplicity
         IceUserObject userObject = (IceUserObject) currentNode.getUserObject();
+
+ treeNodeDiv.setAttribute(HTML.TITLE_ATTR, userObject.getTooltip());
+
         // a branch node is a node that is not a leaf
         boolean isBranchNode = !userObject.isLeaf();
         boolean isExpanded = userObject.isExpanded();