Taking a shot in the dark here, b/c I haven't looked at the code, but I know this is case for Sencha Touch.
Part of Ext 4's direction is to eliminate the modification of the built in objects e.g. Function, Date, etc. So rather than call createDelegate on the function, you now call Ext.createDelegate(function...
I would imagine that when v4 is released, this will be addressed by the compat layer to avoid breaking old code.