Also, I didn't realise but watching that Mark Cerny video, they seem to have announced that the SSD has 825 GB of space available.
I think we'll be able to have a External HDD like we can now but we have to move the games off of it to play them. It would be interesting if only a portion of the data in each game was saved on the SSD and the rest streamed in from a fast HDD when you start the game. But I guess that ultimately that would undermine the point of the SSD being to give this generational leap so it's probably a good idea they design the consoles