quamsta: you see, I don't know if this is something to fix or on purpose left like this. I would LOVE to have it included in the official distribution.
JamesS: Thats basically the script yo would run to symlink all the "dependencies", yes. So far so good. But you mention three new files that should be modified; Image.php (in sapphire/core), /cms/code/AssetAdmin.php and /sapphire/core/model/ErrorPage.php. So thats an additional 3 files. However, I believe after a quick look that modifying the Image.php file (by replacing ../ with Director::baseFolder()."/" as you say) is enough to cancel out the changes we made to GD.php, which is good.
What I don't like about all of this is that as it seems right now we would need to modify 4 files in the sapphire folders and 1 in the cms folder... Having to modify the core of the system each time an update is released and applied doesn't seem so good...