A DVD-VIDEO disc is NOT a simple data disc like your CD-Rs with files just thrown on them
For a set top player to play video on a DVD it must be in the correct format and hierarchy.
A DVD authoring program, like iDVD, does this for you.
Notice on a storebought DVD there is typically two folders on it:
VIDEO_TS
AUDIO_TS
inside the video_ts is .vob and .bup files etc.
contained within these odd files is your video, chapter markers, menus etc.
You can not simply put a movie file on a DVD, like a CDR, and expect it to play on a set top player.
Yes .mpg is an acceptable DVD video format, but it has to be authored to create the correct files for a set top player to know what its doing.
DVD videos are NOT as simple as many people think.
iDVD makes it simple though.
having your video in the correct "format", like .mpg, just means the video doesnt need to be converted/rendered. they still must be authored though.