This is due to the fact that the Ubuntu devs have recently enabled RGBA transparency in the new version of GTK+. See this IRC chat with bratsche:-
<bilalakhtar> Will RGBA transparency be enabled by default in Maverick? if not, will its support arrive in Maverick?
<bratsche> We're planning to land that in the Meerkat distro within the next two weeks or so. There will likely be bugs (e.g., it's known to crash and slow down totem right now still) but we want to get it landed early.
This is due to the fact that the Ubuntu devs have recently enabled RGBA transparency in the new version of GTK+. See this IRC chat with bratsche:-
<bilalakhtar> Will RGBA transparency be enabled by default in Maverick? if not, will its support arrive in Maverick?
<bratsche> We're planning to land that in the Meerkat distro within the next two weeks or so. There will likely be bugs (e.g., it's known to crash and slow down totem right now still) but we want to get it landed early.