I don't believe Apple is ready to take the move past Nvidia GPU/chipsets. We keep hearing that Apple is planning to use this Nvidia chipset for a long time to come. When Apple does switch the 13" MBP, MacBook, and Mac mini, the last of the list is typically the MBA.
I would say end of 2011 would be a good time to upgrade as long as the 13" MBP, MB, and Mm have all been updated it would then be upgrade time.
However, don't expect all of the upgrades you want at a $999 price point. Apple could have made the 11" MBA standard with 4GB RAM, and larger SSD, but then the standard starting price point would be more expensive like $1299. Apple essentially downgraded the model to allow the $999 entry point.
There is always hope, but I don't see a backlit keyboard in 11" MBA for its next upgrade. I think the 13" MBA is likely to get it but not 11" MBA. There is hope for a 1.86 GHz or 2.13 GHz 11" MBA with same C2D CPUs.
I think a lot happens but it's a long ways out. Apple has shown its willingness to stick with old irrelevant technology and components for a long time just making their Macs look pretty. Hell, it could be two years more before they eliminate Core 2 Duo... I don't think it will be two years, but with their track record, these exact MBAs could stay the same until holiday season 2012.