Gimp currently supports only PNG24 and PNG8, but when exporting a PNG8 with an alpha channel it will be treated the same as a GIF image with a 1bit per pixel alpha channel.

Unfortunately the Wiki entry about PNG [1] seems not to contemplate this specific case although it has been documented in other places [2]

I'm currently uncertain of how the alpha will be technically stored if either as a separate channel or within each pixel, see [3].

As [2] suggests PNG8 are absolutely great when working on the web and such feature would benefit many users of The GIMP.

Thank you very much for taking this into consideration.

[1] http://en.wikipedia.org/wiki/Portable_Network_Graphics#Color_depth
[2] http://calendar.perfplanet.com/2010/png-that-works/
[3] http://www.libpng.org/pub/png/spec/1.1/PNG-DataRep.html#DR.Alpha-channel