I've been considering it... I don't know if so few people have it because nobody dares to try, or because it's actually hard. Apparently the last F.Mus was like, 5 years ago, and "it was just some stupid horn player or something, pffft" - my (elitist) teacher 
It's just an 80 minute recital (http://www.ameb.edu.au/client_images/678246.pdf) - no general knowledge or theory, thank god. But I have no idea how high the standard actually is, and nobody I know has done it so I can't ask... plus I bet it's really really expensive and my parents refuse to pay for "such a waste of time" (i.e. anything music-related) so it'll be coming out of my own pocket...
In my conferring ceremony (2009), there was a guy who got an FMus in Trumpet (Christopher Eury). He performed in the ceremony and could make all these awesome noises with it
Yeah, I have no idea about trumpet playing at all!
On that note, I heard that this year's diploma conferring ceremony had three performers
If anyone went there, who/how were they?
I only know of the cellist...
is there actually a difference between memorising and playing with the music there but not looking at it? 
in my opinion, there should be some slight difference, but i think its safer to play with the music despite having it memorised in case you forget 
Honestly, for these diplomas, you will have played it enough to be able to play it without music.
Although it is 'safer' to play with the music in front of you, it gives you the temptation to look at it (not good!

)
If you're worried about not being able to memorise it, then perform the pieces a few times, maybe MSV etc.