After discussion with Symantec about this a few months back, I did get an answer. A weak one,but better than nothing, I guess.
You have to specify the exclusions at or inside the level in the directory where the includes are. I.e.
If you're backing up:
e:\staff\home\*.*
e:\shared\*.*
e:\2008\*.*
and you try specifying an exclusion of e:\*.mp3 /subdir, it won't work.
You need to exclude:
e:\staff\home\*.mp3 /subdir
e:\shared\*.mp3 /subdir
e:\2008\*.mp3 /subdir
This really stinks when you've got a ton of folders being backed up.
Alternatively, I suppose you could mark the entire drive for backup, exclude the folders you don't want, and then define the filetype excludes at the root of the drive.