But really why would one need it? Soooo much!!!
I'm with the rest -- 2 GB for a camera and get more of them if necessary -- but a big CF card is a dandy way to get stuff from a laptop to the desktop. My mail program (Pegasus) will even reference a CF card (or other movable storage) for the "Mail" folder where all the current and archived e-mails are kept.
16 gigs doesn't seem much nowadays, but it's almost as big as the HD on my older laptop. I'd get a 16 gb CF card in a minute if it were $69, and that time may come, especially if a few more people wake up to how much more convenient (and from what I've seen faster) a CF card and carrier or USB reader is than a "keychain" drive or whatever they're calling them now -- and plugging in a cable doesn't wreck the USB socket like rassling a rigid drive into it will. Or maybe that's "doesn't wreck it as fast."
By the way, I'm storing an extra CF card in the space where the 2nd battery would go in the 30D's battery grip, have been for months now, and it doesn't seem to hurt it being next to the battery. (That I thought it might shows how ignorant I am of how a CF card works but I'm sure someone here will set me straight.)