As a previous poster had suggested, it's bad form to touch things in /home/USER during installation. As mentioned there isn't a Global Desktop that you can add it to, so what happens if there are multiple users on the system? Or a user is added after installation?
That said, I believe you could do it in postinst by copying the .desktop file to the /home//Desktop/ folder. You would also want to have a prerm (or perhaps postrm) to remove the file during uninstallation. Verify the file is there before removing in case the user deleted the desktop icon (Also, what happens if a different user uninstalls it?). At this point, this is getting pretty messy.
Technically speaking, that should work. I'm just not sure if you would be able to get that past REVU to be included in the official Ubuntu repositories.