If they go Sandy Bridge, maybe as early as October. I don't believe it would be wise to use the low and ultra low voltage SB IGP in the MBAs. The IGP is seriously inferior, and you cannot use the standard voltage IGP used in the 13" MBP as an example as it's running full clock speed. I don't think people are thinking this through.
I believe Apple has a problem as the IGP in SB isn't all that especially in low and ultra low voltage chips. I would prefer to see Apple finally bring on AMD for CPU and GPU in the low end Macs. The MBA is the only Mac that needs low and ultra low voltage chips. I suppose the real problem then is Thunderbolt.
I feel like the best opportunity for a realistic MBA update is Ivy Bridge in January 2012. In addition, Apple doesn't update its Macs in a cycle that would allow an MBA update in June or even before October. Most importantly, everyone that owns a current MBA is fully aware that it doesn't take a new chip to make an amazing Mac, but there are many suckers out there that believe Intel's marketing BS... the way to make the best Mac is not to update the CPU and incorporate an inferior IGP to the Nvidia GPU, it's to use a better drive and drive controller and use software making instant on and take advantages of the battery capabilities and etc of Flash storage. The MBA is the fastest current Mac for the average user not running CPU intensive apps, and it has a C2D.
We also have heard many saying the Nvidia 320m will be used for a long time to come. It made some sense to update the 13" MBP to SB because pros need the CPU for a few professional apps that need nothing more than CPU horsepower. It makes zero sense for Apple to use the inferior IGP and possibly tarnish the MBA's reputation just so the very few that believe that the only way to improve a computer is to listen to Intel's marketing BS.
I want an update to SB if Apple can use a 7W discrete IGP (edit, mean GPU) and run the CPU on turbo all the time by disabling the IGP. It would bring the best of everything. Apple could get away with using an ultra low voltage SB even in the 13" MBP but get better clock speed. This is the rumor we heard about a year ago, and to me it's the only one that makes sense. I would love for this to come with Lion in June, but any SB that forces Apple to use the SB IGP as the sole graphics solution seems senseless to me. I also love the idea of Thunderbolt and a backlit keyboard.
Finally people that want to think about quick updates to the MBA are hoping but not using the MBA and Apple's track record to look at history to make an actual prediction. Apple used the exact same MBA for two years with only a spec bump to the prior same cost CPU bumping only the SL9x00 series. We really had the same MBA from October 2008 to October 2010, so why would Apple bump after eight months when it would require seriously altering its capabilities and possibly negatively affecting its brand when people start having problems related to the loss from a real GPU to SB IGP.
It just doesn't make sense... And when it doesn't add up, I believe it's smarter to follow the history than go with hopes and dreams of a few to disappoint the masses. I only want SB if it means a better complete system, and I don't believe Intel's IGP running at a much slower clock speed than even the disappointing 13" MBP would be an improvement in any way for the MBA... Long live the C2D and Nvidia GPU until Intel can actually beat the technology all around. If Apple can use standard voltage SB in the MBA, maybe we will see a SB CPU, or if it can use a discrete GPU with ultra low voltage CPUs, then sure. Otherwise I think Apple needs to wait for Ivy Bridge or a complete AMD solution.