Now that you are rooted you could delete /system/media/Droid.ogg. You will need something like Root Explorer to access that folder and I would recommend renaming it Droid.bak just in case something goes wrong, but its an audio file so I don't think it will cause any harm.
I really want to do this as well once rooted, its so annoying that this phone still plays that even when I'm in vibrate mode. Even my old feature phone knew not to play it's start up sound when it was in vibrate mode.