Comment 3 for bug 336332

I think I tracked down the issue and it's just like Troy suggested in the original report.
In 0.48 PNG images (from imported/exported/bitmap copies) work fine, and alpha compositing is performed as expected.
Tiff is still a problem. Apparently because Inkscape assumes that every imported RGBA bitmap will have straight alpha, so it premultiplies the alpha for alpha compositing.
According the specification, tiff files are stored with premultiplied alpha. If inkscape premultiplies the alpha channel again, the result is a double premultiplication that looks like a dark halo around the blended areas.
This bug makes working with any alpha-premultiplied image format impossible and it will be critical in some environments.

Considering how frequent premultiplied alpha channel is (mostly in images from 3D packages) there should be a workaround. Assuming the alpha mode doesn't work always, so probably a toggle in the import dialog allowing to switch between alpha modes would be the most flexible solution.