Actually, I think this would be a pretty neat idea. Gives RockBox a sense of class, and can act as a simple test to ensure everything is working.
Would anyone be interested in making this? I suppose a decent track/sample could be found from places such as TindeckAudio and (with the author's permission to modify it) edited and dubbed to include a voice-over.
What would be a decent script? As above, I think something along the lines of the following would be sufficient:
"Welcome to RockBox, the Open Source Jukebox Firmware. This file confirms that your player is working normally under under the new firmware. Please be sure to visit the RockBox website and read the RockBox manual before continuing."