Up to 8GB on the Macbook Air? Never going to happen. It's not a pro machine and it is not designed for pro use. If you are running something so intense that it requires 8GB of RAM then the MBA will probably melt.
Anyway, I think the iMacs will be the next major thing, though of course iPod updates and Apple TV may come before or at the same time.
Notebooks may get another touch up before the end of the year (around November or so, just in time for the holidays) or beginning of next year.
***** Although the one possibility that not many people are discussing is that Apple will intro a whole new regular Macbook line (this is different from "updates" to existing products). Somehow I don't see them keeping one configuration of an old design (good, but still old) as the only option for those who want a non-Pro Apple laptop.
I wonder what they will do there. Perhaps this will be Apple's answer to the whole netbook thing. Maybe they will introduce 2 sizes of new Macbooks, an 11" (which would still allow for a nearly full size keyboard if not a proper full size keyboard) and a 13" to replace the existing one. Of course they wouldn't be netbook priced, but would be thin/light and allow themselves to be as convenient.
Or maybe they will drop the Macbook Air and make that the standard Macbook. They just did a $300 price drop, so obviously their costs have decreased enough to make high enough profit margins in order to do that, so perhaps in the next few months they will drop it further and count on the economies of scale to keep the margins high enough for Apple's standards.
Whatever they do, I think they will definitely add some variety to the mix. Having 6 different options for a stock Macbook Pro (two 13", three 15", and a 17") and only 1 option for a regular Macbook looks just ridiculous.