I would say that just because it's the only one like it right now doesn't mean it will not become a third-party market option later. Look at Runcore, PhotoFast, and others. The LIF SATA-II was almost a proprietary type solution... at least it was only used by Apple but others made SSDs that were faster and had more storage than the stock SSD even.
Since it's "plugged in" to a proprietary port, and not soldered to the board, leaves the option for expansion by third-party vendors later. The thing is Jobs made it sound like this MBA is the future of all of the Mac notebooks. I suspect probably other Macs will get NAND Flash even if it's just the main storage holding OS X and apps. That makes more sense for the bigger thicker Mac notebooks to give them both 2.5" SATA drives for file storage and NAND Flash for OS and apps giving it extremely fast startup, app opening, and etc.
If Apple does use this tech in other Macs, it's almost guaranteed that third-party suppliers will make them and sell upgrades.
Now, with the RAM situation, it's soldered to the board, and without an expensive robotic arm in a research or factory environment that isn't upgradeable at all for the common consumer.
I would tell people to upgrade the RAM to 4 GB no matter which other options they want. While I fully believe 2 GB RAM is sufficient in OS X for the current situation and majority of owners, to future proof the machine 4 GB RAM is very necessary. Right now it's necessary for those who want Windows, but apps will require more RAM, and Lion will probably require 4 GB by itself, and the RAM is shared as VRAM for the Nvidia 320m.