After several generations of netbooks, people are still pining for optical drives for the one or two times a year you wish you had one? I'd much rather the smaller form factor and a USB key for data transfer.
With regard to slow networking, I wonder if a gigabit ethernet/USB adapter like the Belkin F5D5055 would help. Obviously USB 2.0 maxes out at around 30-35MB/s, but if the adapter can actually attain those speeds it should be fast enough when you need a bit more than Wifi.
Generally, I'm very tempted to go for the maxed-out version of the 13" MBA. I'll probably take a look at the screen at a retail store though, I'm not keen on the glossy finish, and my old 2006 MacBook has terrible angle dependent colour representation. Going to make this the motivator for launching my first product. :)
Still, that's 3-4 times slower than USB (in theory).
That said, there are existing solutions for this, they just suck, e.g.:
http://www.seagate.com/www/en-us/products/laptops/laptop-hdd (only 4GB Flash, WTF?)
http://www.bit-tech.net/news/hardware/2010/02/03/silverstone... (this is just a dumb partial RAID)
http://en.wikipedia.org/wiki/ReadyBoost (designed to work with USB sticks of all things, and isn't terribly smart about what to cache)
Plus, that sort of file-based system doesn't cache file system structure/metadata, which is a major source of disk head movement.
If you only store your OS and programs on the boot drive, space isn't a big concern and you gain a lot of flexibility. It's less hassle to repave the OS, migrating your data to new machines is simple, and it's easier to upgrade the boot drive as faster ones are available. That's especially nice with SSDs because you no longer have to care very much about the write limit. If my SSD boot drive wears out, I have 0 data recovery/restoration to worry about.
I'm thinking of it myself, but I also thought it out further and concluded that it could be completely automated. In my case, if one just kept all files larger than 100MB on the HDD, that would be enough. Better still, I could specify certain directories to be kept on the HDD. I could do all of the above with symlinks, but setting that up once and getting the desired result automatically could be even better.
The optical drive has been very much neglected by Apple: there is no Blu-ray drive in any Mac they sell. Removing all optical drives in the next year or so certainly wouldn’t surprise me. (I actually hope that they will do just that. I’m going to buy a 15" MBP next year and I sure would like it to be a bit lighter and with a somewhat bigger battery.)