The Air will probably be ARMed this year. At least do the transitional model like the retina MBP and still offer the Intel version to test the waters.
Maybe I'll be proven wrong, but I have strong doubts about any ARM-based Macs coming any time soon. They're pretty powerful devices, but once you ramp up the processing power to comparable levels, most of the benefits as far as heat and power consumption disappear.
I doubt we'll see ARM-based Macs anytime soon. Windows RT isn't exactly setting the world on fire, and Intel is getting their act together on power consumption. Although yesterdays "7W" Ivy Bridge processors turned out to really be 13W (7W is the average, not peak consumption), it's a start, and Haswell should make true 10W processors a reality.
That said, I do think that a future MacBook Air will be thinner and lighter, though probably not in 2013. Similar Ultrabooks have gotten under 2.5lbs now. The rMBP is probably set for now, but I also believe it is a target to get that under 3lbs eventually (maybe the first major redesign, which isn't likely before 2015).