Many of us who bought 2010 Airs are in this position, and the right decision for me isn't necessarily the right decision for you.
A good rule of thumb for many people is this:
Unless your 2010 air isn't powerful enough for the kinds of tasks you throw at it, keep it until the 2012s come out.
There are few benefits to upgrading to the 2011 air: you get a much faster processor, a backlit keyboard, and a thunderbolt port.
The backlit keyboard is very cool, to be sure, and faster is always better. But Airs will likely get faster with each new model, so if you don't need the extra speed right now, you'll do even better by waiting until the 2012 model arrives.
Thunderbolt is important if you want the new $999 Thunderbolt display. If not, you can almost certainly live without Thunderbolt for another year. There are very few Thunderbolt compatible accessories at this time, and they tend to be very expensive.
There are reports that battery life on the 11" Air is maybe 30 minutes less on the 2011 model than on the 2010 model, and the 2010 model's NVIDIA graphics is actually better than the 2011 model's sandy bridge graphics. (Though for most of us, the 2011 model's graphics should be perfectly adequate.)
The 2012 model may well have other advantages that we cannot yet predict. Maybe Apple will squeeze another 30-60 minutes of life from the battery, or have more and cheaper SSD options, or use a faster SSD. Or maybe USB 3.0.
(That doesn't seem likely now, Apple seems to be purposely sticking with USB 2.0 to encourage people who need speed to get Thunderbolt devices. But a lot can happen in 9 months, and if Thunderbolt accessories remain scarce and expensive, who knows, maybe Apple will give in and put USB 3.0 ports on MBPs and MBAs.)
Finally, I suggest you do a bit of math. Look at what 11" Airs are fetching on eBay right now, and subtract that amount from the price of a 2011 Air. That's how much it'll cost -- and there's always a risk your Air won't sell for as much as you expect. If that cost is worth the benefits of upgrading now, then go for it. If not, wait until the 2012 models arrive - you'll be glad you did.
For me, the cost of upgrading would have been too high to justify a speed increase that I can live without for another year. Plus, I'm working crazy hours now and you have to budget time for selling on eBay and moving everything over to a new machine, that would have been tough for me right now.
Good luck with whatever you decide!
