Comment 0 for bug 372514

.desktop files are used for Menu entries to launch applications, and are managed by Ubuntu packages. In this case, absolute pathnames are ok, since the .desktop file exists in a specific directory and the absolute pathname of the executable is known from the package, etc.

.desktop files are also used for Launchers, however. In this case, it would be very useful to allow relative paths (for both Exec and Icon), so that the Launcher can be bundled along with an app that doesn't have a .deb package, and still be launched easily.

http://<email address hidden>/msg04553.html
http://ubuntuforums.org/showthread.php?t=1037480