Here are some things I've learnt since my last post:
With less than 99 photos you can make a slideshow with iDVD (just drag an album and song or playlist - which can contain more than one song - from the Media panel)
iDVD treats each photo in a slideshow as a chapter and therefore gives problems when you use more than 99 photos. If you want more than 99 photos in your slideshow use iPhoto. You can join songs in Garageband or Join Together, reimport them to iTunes and use that one, extended song as your soundtrack in iPhoto.
However...protected songs gave me a problem. Here's the workaround I used when I wanted to join songs, one of which was protected:
1) Make sure iTune Prefs (Advanced > Importing) are set to 'On CD Insert: Show CD'
2) Make a new playlist of the tracks you want to join.
3) Burn the playlist to a CD (or CD-RW).
4) Eject then insert the burned CD and iTunes will open.
5) Within iTunes, select the tracks on the CD that you want to join and then from the top menu select Advanced > Join CD Tracks.
6) Then select Advanced > Convert Selection to AAC.
7) Put the new combined track (you can get it from Recently Added) into a new playlist.
8) In iPhoto, select this playlist for your slideshow.
Hope that helps.