Honestly I'm at the fingernail-chewing point where if they just released a refresh with the SL9600 CPU and 256GB SSD, I'd bite without hesitation. Anything else of worth (for me, 4GB RAM and battery tech only, not in the slightest interested in the extra glass others seem to crave) would just be a nice bonus.
I think you'll be lucky.
My predictions after consulting the crystal ball:
SL9600 CPU: 99% probability.. Apple has, with only very few exceptions, always upgraded the CPU. The minor speed bump will move the MBA above the psychologically relevant 2Ghz threshold, too.
256GB SSD: 99% probability. The 256GB SSD from Samsung offered by Apple for the MBPs is also available from Samsung in the 1.8" form factor.
4GB RAM: 70%. Similarly, RAM prices are down considerably from the level of last summer so 4GB would also be possible. If Apple will not offer more memory, then the fact that upgrading RAM is not possible would make buyers more and more hesitant to pay the "apple tax" in a time of economic crisis, especially since netbooks are moving towards 2+ GB.
glass trackpad/black bezel: 50%. I'm sure Apple is trying to fit those in. If they manage without compromising size or weight then it will be done.
15" MBA/16:9 ratio/OLED: 25%. If the OLED rumor is true and that 15" LG panel will be used in an Apple notebook this would pinpoint to the introduction of a 15" MBA and a shift to a 16:9 ratio for the Macbook Air series only. The resolution of the OLED display (1366x768) is too low to be used in a MBP and the 16:9 ratio could be a "feature" exclusive to the MBA. The 13" model wouldn't have an OLED display and for the 15" it would be BTO. Even if the OLED won't be used we could still see a 15" MBA soon - there was a page 2 rumor about this a while ago...
increased screen resolution: 25%. There is currently no proper 13.3" panel available, and especially not from Apple-partner LG that has a 1440 resolution and delivering acceptable quality. The one used in the Lenovo x301 is crap. Similar situation as with moving the 15" MBP to a 1680x1050 resolution. But should there be something available, I think Apple might consider it also to set the MBA further apart from the consumer Macbooks that might cannibalize its sales more and more since the unibodies are lighter and of higher build quality. This speculation is not compatible with the OLED rumor above, however.
carbon-fiber bottom plate: 25%. There was a rumor about that a while ago. As with the glass trackpad and bezel, I'm sure that Apple is likely to be working hard on that and if it can be done it will.
firewire-port: 10%. With the MBA further developing, Apple might find space to include a FireWire 800 port especially for fast storage solutions. I would welcome this but I fear this is more wishful thinking than anything else.
Rev. C / MBA II release date: May/June 90%. The main reason Apple held off for so long (273 days compared to around 190 for MB/MBP average) with Rev. B was that the components (CPU, 120GB harddisk) were just not available earlier. Also, since the new iPhone will be introduced at WWDC, it would be convenient if Apple then would focus on the subject of mobility in general. Introducing a new MBA and/or the rumored netbook/tablet would then be just fitting.