New NGUI releases always support only the latest Unity versions officially -- latest Unity is currently 4.3.4 in Unity 4 cycle, and Unity 3.5.7 in Unity 3 cycle.
I can't test older versions of Unity as I don't have every single version of Unity installed (and it wouldn't make much sense to do that anyway).
I will add an #ifdef around this tomorrow, but keep what I said in mind for the future, as this is bound to happen again.
Rule of thumb: the latest version of NGUI requires the latest version of Unity.