Scottsdale -- First, I agree that a 10 hour battery probably isn't in the cards for the MBA, even if Apple should opt to make it slightly thicker and heavier. It would be nice, though, if there were a way to increase its very limited 5 hour battery to 7 or 8 hours. Obviously, though, if that would require increasing the MBA's weight significantly, Apple probably wouldn't do it, and shouldn't in my humble opinion.
My primary point of concern with the MBA, as you know, is its hardwired 2Gb of RAM. Frankly, I don't think even 4Gb of hardwired RAM would be enough to convince me to buy one. If RAM slots could be included, though, even at the expense of a slightly thicker, heavier computer, I would almost certainly buy one. All of this gets back to what we see over and over again in laptop design. Everything is a compromise, with the need for thinness and lightness battling with the need for power. Every time I contemplate this dilemma I come to the same conclusion: there is no free lunch.
I seriously doubt Apple would ever consider going thicker in any way than the current MBA is. In Apple's history of selling Macs, it rarely changes a model/case design in less than three years. I believe we're going to see the "next" MBA case design in January 2011. The question is, will we see a new MBA with the exact same case one more time before then. With Apple's strategy of using one base configuration for five Macs, and with Apple sticking with C2D and Nvidia GPU/chipset in the 13" MBP, I don't think there's any reason not to update the MBA and keep it relevant until the major update next year.
When the MBA gets updated, I agree that Apple might eliminate the tapering. Look at the new iPhone HD design to see what Apple could do with the MBA. It could go thinner in the thickest part and use that one thickness across the entire MBA. But will that be better? I don't know, because the MBA feels like it's 1/4" thick due to the tapering. It might feel like 2/3" all the way across which could feel a lot thicker than the current MBA. With uniform thickness, there would be far less wasted space. However, if anyone takes apart the MBA, they will realize almost zero wasted space in it.
I don't know where Apple is going to take the MBA, but I believe that we will all be seriously disappointed if there's a 10-hour battery in the next MBA. The rumors of an ultra low voltage Arrandale CPU seem baseless given the past and current strategy Apple has used across five Macs including the MBA. In addition, there is the marketing problem of selling a 2.13 GHz Core 2 Duo and moving to a 1.2 or 1.4 GHz Core i7. I just don't believe it would be easy to sell an "upgrade" to a CPU that loses 40% of its clock speed. I still believe when Apple moves beyond C2D it will move to the replacement CPU for the SL9x00 which is the low voltage Core i7-6x0LM. At 25W, Apple could turn off the HD GMA and save 8W, then use a dedicated ATI solution to get the MBA at 29W or less, and maybe even use a different cooling system to achieve such a feat.
I just have to believe that Apple will keep the MBA fully primary Mac capable. Moving to ultra low voltage or sticking us with Intel HD GMA wouldn't be that far from just selling us a damned iPad. Surely Apple wants to keep these products as different as possible to ensure the luxury price tag on the MBA. One last thing, I truly believe we're in for a price update that adds as much as $300 to the high-end MBA. A $1999 high-end MBA seems most likely, and I believe Apple will reward those willing to spend more money with a larger SSD and some other advantage in spending more money... and I hope it's an HD IPS display. It just makes sense in charging a premium for such an MBA, and I think Apple will sell more MBAs than ever before if it just gets the MBA to 4 GB of RAM and 256 GB SSD. I would love two RAM slots, but I believe we will have to wait for a newly designed MBA to get such a luxury.