I have a 4g and 2 weeks ago I had a problem with the top half of the touch screen not working. When I went to the Genius Bar it was replaced by a 4g in a box. I asked what happens if they run out of 4g's for people having problems. He told me not to worry there were plenty of 4 g's they have for replacements. I guess a lot of them are problem ones that were refurbished although he said I was getting a brand new one, but I am not sure I did because the battery had usuage on it when I used it the first time. Eventually though( 6 months) a lot of warrenties will run out and they won't need to replace as many 4g's. They are real confident they will have enough 4g's for replacement. At least thats what my "genius" told me.
I was hoping they wouldn't have any in stock and I could pay the difference for the 8g but they said I couldn't do that, it had to be exchanged with the same, that goes for ipod's too (i.e. if you have a white and they only have black instock they will not exchange it, it has to be same color too).
They told me if the Apple store doesn't have a 4g to exchange with yours in that case they said you would have to go through the 800 number and exchange it by mail, and if I had a problem in the future to call ahead to make sure they had 4g available for exchange at that store.
This was my experience, I have come to learn different Apple stores handle things in different ways, but IMHO generally I don't think you will get an 8g unless you buy it.
They will send you a refurbed iPhone. Sometimes Applecare and Manufacturers alike will tell you you are receiving a brand new unit and are either embarrased to admit or just tired of listening to people complaining about refurbs. They are in fact refurbished, everything that Applecare sends out.