I believe the Nvidia-based current MBAs will stand the test of time better than the next Sandy Bridge MBA with Intel IGP.
Just think where computers and Macs are headed. Now consider quadruple the resolution in Lion for Retina type displays in Macs. Also consider gaming, 3D, HD, GPU capable apps, and consider how bad the Intel IGP will be at less than 1/2 the IGP capabilities of the 13" MBP. This is disaster!
I would be happy to use an Nvidia MBA for four years while the SB IGP will make the next one obsolete and behind the times the day it's released. When a computer has a slower CPU it does certain things slower but still does everything the faster CPU can do. With Intel's forced IGP it simply will NOT function for the types of things users want to do. I really don't understand why Apple is going to accept Intel's IGP forced upon it at a horrific cost to its users.
I would prefer using today's MBA and for sure maximizing 4GB RAM on it to last a strong six plus years. There might be 512GB SSDs to upgrade to, but it will otherwise be more capable than Sandy Bridge, Ivy Bridge, and Haswell (I think that is the successor name). It is really sad, and I only hope that Apple tries to make up the terrible IGP with 8GB RAM or two RAM slots, a 512GB SSD, backlit keyboard, USB 3.0, Thunderbolt, and a bunch more features with a price drop! That is the only way I believe they can justify it, and the usability for games and such will be worse than a first gen iPad - maybe an exaggeration but you get the point.
Either buy now or buy clearance once the new MBA is announced, but if you're thinking three plus years out you must get the Nvidia GPU to ensure yourself some capabilities.