Yeah, but you have NO IDEA what that cost is.
Apple says they'll have to sell them for $699 unlocked. AT&T sells them for $299. Does that mean AT&T gave Apple $400? Heck no. There's some magic number between there that they pay, and netiher you nor I no what that is.
So what DO they pay them? What if it's $100? (As a random guess) What if AT&T lets 3G users upgrade if they pay that extra $100 and extend their contract 1 year? Then AT&T gets some cash to cover the fist phone, and they've just locked you in for one more year which will easily make up the cash for the 2nd phone. It's win-win for them in that case.
So in that case, an upgrade option is VERY likely! Now, I have no idea what the magic number is, so I can't say that this will happen. But it's also true that you don't know what the magic number is, so you can't say it WON'T happen!