Why is the default background image still named /usr/share/backgrounds/warty-final-ubuntu.png?
It's not Warty,
and it's not even a PNG:
$ file /usr/share/backgrounds/warty-final-ubuntu.png
/usr/share/backgrounds/warty-final-ubuntu.png: JPEG image data, JFIF standard 1.02
Because that's how their first default wallpaper's filename was. They wanted to make sure that whoever wants to stick with the distro's default wallpaper would get updated to the most recent version.
Since the wallpaper setting is stored in a per-user setting in gconf, it's not possible to assign new wallpapers to users on upgrade, as they (rightly so) don't want to override users' wallpaper settings.
Hence, the only way is to leave the filename alone and change the file the settings point to.
Another reason people may care is that for some scenarios, you'd like to change the default wallpaper using command line tools only. It's just very unintuitive to find that you have to
mv $MYBGIMG warty-final-ubuntu.png
instead of using something like default-wallpaper (without extension, while we're at it).