Sorry to argue semantics, but there's no such thing as "definitely". Very likely is about as far as you can go.
I think 4GB RAM is actually less a priority than storage space, a faster CPU, and better battery longevity. The proportion of those actually maxing out at 2GB RAM under normal daily use even using 'Pro' apps is relatively small. Even if you do hit the RAM ceiling, it's always a user-manageable situation rather than say choking due to a slower CPU.
With the OS/software focus these days focussing on streamlining, making things leaner and less resource-hungry, I don't see it personally as a deal-breaker for me as long as the other stuff is present in a rev.C. Then again I would never dirty my MBA (or indeed any other Intel Mac) with a Windows partition.
I will take a 1.86 GHz Penryn 17W SL9400 with 4 GB of RAM before I will take 2.13 GHz SL9600 with 2 GB of RAM.
I think the 4 GB of RAM, at least in high end MBA, is definitely if released in June. I think that the longer before an update, the more likely a new case, and if that anything can happen like lower powered and more portable MBA.
Apple has doubled its RAM offering in all its Macs in the last releases. It seems "definitely" to me. Apple cannot ignore the MBA's two biggest competitors the Dell Adamo and Lenovo x301, which both have 4 GB in high end models. And the Dell has an ULV CPU, which I pray to GOD Apple doesn't do. I want the SL9x00 CPUs.
In fact, I will take a rev B MBA right now with 4 GB of RAM and no other changes before all of the other changes and 2 GB in a rev C MBA. Really, RAM is the number one priority to make the MBA a primary Mac for me moving into the future. I lived with 2 GB of RAM before, and it is main limitation of the MBA.
So, I would take...
1.86 GHz
4 GB RAM
128 GB SSD
Nvidia 9400m
same form with possible lines on LED display
same large aluminum bezel
same 3 lbs
before I would take...
3.06 GHz ULV CPU (just to make a point, no not currently feasible)
2 GB RAM
1 TB SSD
Nvidia 9600 GT/DUAL 9400m
Black Anodized Aluminum
15" OLED
no bezel
Glass Trackpad
2 USB ports
1 Firewire 1 eSata 2 mini Display Ports for dual external graphics like MP
Integrated BLURAY Optical Drive
1/4" thick
1.5 lbs
Unfortunately for me, I can work around all of the other things including drive space which is easily accessed via AEBS attached drive, but I need 4 GB of RAM for Primary Mac use moving forward. I thought it was ok in the past to get by with 2 GB, but this is 2009 and 4 GB is needed for use as a primary Mac.
All of those other things are amazing and great, but 4 GB of RAM makes it a primary Mac with all of the apps I want to use and the hardware I am already used to in the current MBA.
In my opinion, the 4 GB of RAM is just that critical. I would love the otherwise perfect MBA, but with 2 GB of RAM it is not a sole computer for me. It remains my primary use Mac, but I want an MBA that can do everything, and I need 4 GB RAM for it to do everything for me.