For the record, I never thought 8GB would be enough for me but it's been serving my needs just fine thus far. I have a ton of videos, but I just rotate new ones in before I go to bed or whatever. If you have a ton of music I could see the problem, but I have about 1,000 songs, most of my photos and two or three TV episodes on there at any one time and there's still a couple of GB left over.
when i got my 8gb, i thought it wasn't going to be enough. however, i still have 2gb left over on it. i've realized i don't really use the ipod that much. when i go to the gym, i rather use my regular ipod because i don't have to be as careful with it.
There are stories about the iPhone and iPods causing a flash storage shortage. So my guess is that it's going to be a while - until the market starts producing more flash memory, which would bring the price/MB down.
Also it would make it easier for apple if 16GB took up no more physical space than 8GB - so there's room for it in the iPhone. I don't know if that's an issue though.