PSA: Don't Buy A Macbook Air (Yet)
There are some big upgrades coming to the Airs that I think are worth waiting for. If you need a computer now and the Airs are the best fit, of course, get one. They're great machines. If you are the type that upgrades they're laptop every year, this won't matter to you either (and I want to be your friend). But if a purchase isn't urgent and you, like me, keep your laptop for 3 to 5 years I recommend waiting, at most, until the 2014 update.
Redesign: It's been over 2 years since the Airs have been redesigned. This isn't a problem as such but they're starting to look a little dated next to the Pros. Rumours have it that a redesign is coming this year. This year or next, I bet one will accompany the retina displays.
Retina: I think everyone agrees that the hold up is battery life. The screen tech and processing hardware have to reach a level of efficiency for Retina to work in an Air. The good news is that Haswell is just what the doctor ordered.
Haswell: If you could design the successor to Ivy Bridge specifically for the Airs, you would have a hard time coming up with something better than the Haswell platform. With Haswell, Intel will trade CPU enhancements for greater power efficiency enabling much better battery life (and Retina displays). And although the CPU performance won't see a huge boost, GPU performance will (2x is expected). An enhancement the Airs cry for.
802.11ac: Simply put, real world performance is 150Mbps for 802.11n and 450Mbps for 802.11ac. Rumor has it that Apple will introduce the upgraded Wifi this year but that was also the rumor last year. Regardless, it's coming soon and you will want it.
Thunderbolt 2: Code named "Falcon Ridge", Intel is touting double the bandwidth. Current Thunderbolt is capable of 10Gbps of bidirectional bandwidth, version 2 will manage 20Gbps. Do you hook your Air to an external display? Thunderbolt 2 will finally provide enough bandwidth to enable a retina Thunderbolt display. Falcon Ridge is expected next year.
LTE: I'm 100% sure all the prior upgrades are coming but I can't say the same about LTE. Out of all the upgrades, this one is the most speculative. The biggest stumbling block seems to be optimizing OS X for mobile data. You can't have iTunes downloading something huge in the background when you're out and about on LTE. Apple knows people want this and it just makes sense. Look for mobile internet optimizations in OS X 10.9, I doubt Apple will advertise them (unless they also announce LTE laptops at the same time) but they may be there. Also, Qualcomm made Apple's life much easier recently by creating a global LTE chip. Fewer SKUs = happier Apple.