For all the peaple who have problems with GIMP and Blender to get the Alpha-Channel to work when you save your Image here is one solution that I find out by hazard :-))
In the Render Buttons (F12) turn on RGBA+Premul and save your rendered Image as Iris, that’s all.
It works fine for me with Gimp Version 1.2.3 under Linux, have a look at:
One good reason why computers can do more work than people is that they
never have to stop and answer the phone.