I looked under the iDVD help and found the following:
Looping movies and slideshows
Instead of returning to the menu after playback, you can set slideshows
or movies to "loop," or repeat from the beginning. When a slideshow
finishes, it starts over with the first slide of the slideshow. When a
movie finishes, it loops to the first scene of the movie.
If there is background audio for the slideshow, the audio restarts from
the beginning when the first slide appears.
You can loop any movie or slideshow. Looping is turned on automatically
in a drop zone.
To turn on looping for a movie or a slideshow in the main iDVD window,
select the movie or slideshow button on the DVD menu. Choose Advanced >
Loop Movie or Loop Slideshow. To turn off looping, choose Advanced >
Loop Movie or Loop Slideshow again.
To turn on looping for a movie or a slideshow (including autoplay
movies and slideshows) in map view, click Map. Select the movie or
slideshow icon and choose Advanced > Loop Movie or Loop Slideshow. To
turn off looping, choose Advanced > Loop Movie or Loop Slideshow again.
Looping autoplay movies can be useful for creating self-playing
presentations, such as kiosks.
To turn on looping for a slideshow in the slideshow editor,
double-click the slideshow button in the menu to open the slideshow
editor. Select the checkbox labeled "Loop slideshow." To turn off
looping, click the checkbox to deselect it.
When looping is turned on for a selected movie or slideshow, a
checkmark appears next to the Loop Movie or Loop Slideshow command in
the Advanced menu, and a small gray icon appears in the lower-right
corner of the movie or slideshow icon in map view.
To exit from looping when viewing the DVD, click the Menu button or the
Title button on the DVD remote control. The Menu button brings you back
to the previous menu, and the Title button brings you back to the main
menu.