If they announce it in June and it ships in August that's half a year of NAND flash price erosion. If you extrapolate the current price trend, that means it will cost about $200 instead of $300. How much do hard drives cost Apple? Probably $50. So the increase in price would be about $150. And the markup isn't 100%. It's around 45% (see iSupplie iMac tear-down). So the extra cost to the consumer will be about $215
OK, though it it's a BTO option, I'd expect that upgrading to that drive to be about as expensive as the drive by itself anywhere else.
I wouldn't go that far. Current notebook hard drives don't consume much power or generate much noise, and the typical 1.8" drives used in a subnote are even thriftier with power and don't make much noise either. My first gen MBP doesn't make any audible difference when it's flailing the hard drive, nor does that part of the notebook get very warm.
Even when assuming the hard drive is on and running most of the time (which is not typical), I think I calculated that a typical 2.5" drive would last about 30 hours on a MBP battery.
Current notebook flash drives aren't any smaller that I've seen either. I still have a hard time believing that a 32GB drive would be useful in an OS X system for reasons I've already noted. A shipping Leopard install + iLife, etc. is probably going to take even more space. Apple would have to really slim down their installations.