Adamz - I shoot 4gb cards. My D40 only takes regular SD (not SDHC), so 4GB is the max. I've never filled it up in a single shoot (I try to delete my junk shots on the fly, because I don't want to deal with them later... and I almost always shoot JPEG). If you shoot RAW at 12MP's then your going to need the 8-16 range. If I made a lot of shots, I'd probably prefer the security of having my shoot on multiple cards to the convenience of having one large capacity card... but that's just me.
NikoDoby - People use them as the primary storage in net books, I don't think are quite fast enough to run applications off of, but if you aren't to picky about performance, it would definitely do the trick. I suppose if you did the fast version of raid (stripe 1) you might get something approximating normal hard drive speed.
To be clear, the 2TB is the theoretical limit. It will be quite some time before we actually get there. We won't even have 64GB SDHC cards for a few months. Maybe when the capacity gets up there we could do bracketed RGB shots and not have to deal with Bayer Interpolation. That would be kind of cool.