While you are correct that there´s no room for a bigger battery, I do believe that the MBA don´t use the same battery-techology as the rest of the MacBook family. So there are room for improvement, how much remains to be seen
http://www.apple.com/macbookpro/battery/
I have been confused with the battery specs myself. When the MBPs got the 7+ hour battery, the MBA's battery stayed the same, apparently. However, with the last release, someone stated the battery now was rated for 750 cycles instead of 300 in the MBA. I don't know if that information is accurate or not. On Apple.com, it clearly states 1000 charges for the MBP, but I have found nothing that states that information about the MBA's battery.
Remember with the last minor MBA CPU bump, although still retaining v 2,1, the battery was very slightly bumped. I believe that was more of a bump to ensure EPEAT Gold and Energy Star 5 standards. I don't think that means the battery technology was changed to the same as used in the MBPs, because Apple isn't stating 1000 cycles on the MBA... it isn't stating anything about the cycles which seems like Apple is trying to hide the fact that it's not as nice as the MBP's battery.
I don't know that changing the technology in the battery would add that much more in terms of capabilities. I don't know how much weight could be added if they did that. What trade off is worth it when trying to stay near 3 lbs? What could help the MBA, but I wouldn't want to see, is Arrandale Core i7 CPU with IGP. That's a 25W TDP system whereas the MBA is currently using 29W (17W SL9x00 CPUs, 12W 9400m). That could add some 16% to the time between charges by reducing the power/performance of the CPU and graphics. That's a worthless direction to go from my point of view. I want the power of a LV CPU at a higher clock speed over a lower clock speed ULV CPU to save some battery drain (especially when Intel's IGP is included).
I actually would enjoy an extra hour or two from the MBA on rare occasions, but I don't want to lose power/performance or make it heavier to do that. Bottom line is I can live with the MBA's battery performance right where it is. What I would really prefer is Apple improving the battery with its new technology so the MBA could get the same battery life, or slightly greater, even with a dedicated ATI graphics card. I just don't need more than three hours of charge the vast majority of the time I use my MBA away from its charger.
What does everyone else think? Would you accept a .5 lb add to the weight of the MBA to get an extra two or three hours of battery power between charges? Would you want to go ULV CPU and stick with Intel's IGP at a massive clock speed loss and graphics performance loss just to get a true six hours from your MBA at 3 lbs? This is an interesting topic, because I believe this is one Apple really is focused on... increasing battery life and improving performance between charges... yet most importantly, perhaps, having a really lightweight MBA. At 3 lbs, I am totally happy.
I still believe the MBA's thickness is thin enough. When considering possible changes to weight or thickness, I would much prefer weight savings over reducing the thickness. Meaning, I wouldn't want it to go to .5" thickness and stay at 3 lbs versus staying at .86" to .16" thickness and losing .5 lbs. I believe the form factor is really perfect minus the dead space around the display and keyboards. It could use a much smaller footprint and still give us the same full-sized keyboard and 13" display. Make it 2.5 lbs, reduce the length and width by an inch each, we have the perfect MBA!