A while back there was a lot of talk about Canonical implementing subtle transparency in GTK for Ambiance and Radiance themes - Is this no longer on the agenda? Was there a hold up?
The problem with implementing rgba currently is lying with the applications that run on the gtk+ library. However, with the switch to gtk+ 3 it is likely that Ubuntu will inherit this feature as a default option.
I'm sorry I don't have a link to hand, but the feature was not dropped, just hindered.
EDIT: Here is a link that indicates the possibility of rgba as a feature of gtk3 (which Ubuntu uses as of 11.10).
It seems like it's, at most, a back-burner issue. There are no blueprints from the Natty Narwhal Ubuntu Developer Summit. The last UDS I've found about it discussed at was for Maverick (blueprint here), in which it was dropped from the beta and listed as "POSTPONED" with no mention of which release it was postponed until.