XCF files are not supported in other programs such as EOG and F-spot. From what I've read the complication is that these programs would have to work out a way to display all the layers correctly, so my suggestion is to make an addition to the XCF format.

When an image is saved as .xcf it can save a compressed layer of all the layers put together, this layer will be completely invisible to the user, but other programs such as F-spot could be programmed to load this layer when it meets an XCF file, which will mean it can display the image and no longer needs to work out how to display all the layers.

I presume this would mean larger file size, and longer loading times, which I'm sure a lot of users will be happy with if it gains support in other applications. But, as there may be users who do not want this, when saving a file, there should be a check box which says something like 'save a preview layer (learn more)' with the link explaining it better. Think of it somewhat like a RAW+JPEG option on a DSLR (except in one file), so you have the main image (the normal GIMP layers), plus a preview image (the compressed layer).

Thanks,
Sam Bull