Dano,
It usually takes me a while to catch up with what everyone else is doing, but so far this is how I'm using the SD card: I like to prepare my songs in advance and save all my settings in panel memories. These I save to the SD card using the Performance settings. On the KN6000 I had these stored on floppies, and the first thing I did on the KN7000 was to copy each floppy to the SD card. As I have gone through my songs I have renamed files to more meaningful long file names.
As my computer is in a different room I have not yet tried linking the KN7000 to the computer, but I did purchase an SD card reader (not lucky enough to get one included like our UK friends!). I regularly backup my SD card to the computer using the SD card reader. Last week I purchased a 128MB SD card (Sandisk) and copied the backup of my 8MB card to the 128MB card. Piece of cake.
I do not use midi files on the KN7000 except to listen to what they sound like, and I do that just with by copying the midi file to a floppy. Now I have a 128MB card I'm going to try Alec's suggestion of playing along to an MP3 - that should be awesome, and should raise even more eyebrows in the audience. Maybe a piece with some good singing, provided the singing doesn't detract too much from my playing.
Mike