sweet deal. how do you do alphas with gimp? all the info i've found so far is kinda vague.
Background Stuff
A useful tool is Quick Mask Shift+Q
Paint in greyscale- white is selected, black is unselected. Light grey with a value of 90 = 90% selected
Layer mask.
Layer > mask > layer mask
To edit a layer mask: (in layer tab) Alt+left click on layer mask border turns green) - paint on it in greyscale. Alt+ leftclick to go back to normal layer mode.
Alpha Mask.
To edit alpha directly. (in channels tab) deselect red, green and blue.
You can then edit directly using paint tools AND ERASER.
Once finished reselect red, green and blue channels.
For versions before 2.2.14
You MUST then reselect the layer you were working on in the layer tab. Otherwise you will drive yourself nuts trying to find out why you can't paint on anything.
Another method
errr..

This may be the wrong way to do it, but it's simple.
Once your image is done.
Do a copy visible and create a new image out of it. The same as flattening the image but less hassle - less chance for screw ups.
Then create a layer mask - edit it how i want it then save the file as tga. (Save a copy)
Gimp complains and says You are about to save a layer mask as TGA. This will not save the visible layers. Then it complains some more but saves the layer mask to the alpha.