He also recently commit some changes that enable client-side Lua scripts. These come in two forms: LuaUser and LuaWorld. LuaUser allows an individual user to customize the look of their UI (and probably more).
LuaWorld will be sent along with the map data when you join a server. It will enable map authors to add graphical details or UI changes that were not possible before without modifying the client itself. Here are some screenshots of a modified version of HiX that trepan made. It includes a Lua script that alters how the client renders the map. In this case, it adds ground-level fog to the map, but lots of other things are possible. Lua code could even be used to draw map objects.