False... anyone who ever runs Parallels or VMWare can use 4GB RAM. I think that is probably a majority of serious Mac users -- the people who are going to spend $1800-$2500 on an ultraportable instead of buying a cheap netbook.
Finally, someone who is a heavy/memory intensive user who actually gets it. I can max out 4GB RAM by just working on a large photoshop file, let alone all the other graphics heavy apps I use on a daily basis. Which is why I maxed my MBP17 to 6GB last week, and its been very effective performance wise by minimising and in some instances if I'm judicious about what I have open can completely avoid Page outs and swaps
I spent $4,045 on the latest 17 MBP CTO, and if there was a 12/13 MBP announced tomorrow, I'd order it on the spot too
The Air uses 16 identical 128MB RAM chips. 256MB chips exist (hell, 512MB chips exist -- they're on your 4GB SODIMM) and could easily be substituted if Apple found it worthwhile. I find their decision not to at least sell a 4GB CTO configuration inexplicable. If they offer one within a reasonable timeframe, I'll buy an Air.
I would ACTUALLY be tempted if Apple did this (4GB option). And live with a USB to FW adapter.
And I noticed when I swapped out one of my 2GB sticks for the 4GB stick they were identical in physical size. So its completely feasible a 4GB chip can be installed in the MBA