Take a look at the example .desktop and .service files in the MaemoPad example application from the New Application HOWTO on maemo.org. The key (I think) is in the name of your DBUS service - it needs to be consistent everywhere, and in some places, it's referenced by full "package name" (e.g. org.maemo.MaemoPad), and in other places, it's referenced by just "class name" (e.g. MaemoPad).

I agree that it feels a bit arbitrary, and it frustrated the hell out of me at times. I also had a lot of trouble creating and launching packages that had a dash in them (dashes aren't allowed in service names) - so much trouble that I gave up and just took out the dash entirely.