Why make it out of aluminium? Does it have better heat properties or lighter weight than say some high quality plastics or steel, or any other metal?
I don't say that with any bitterness. I have and love my air, but as I find myself using less and less of OS X, and really just living in iTerm and my browser, the real appeal of the Air to me now is the hardware. If the Ultrabook stands up to the Air in quality, I'm seriously considering the Ultrabook + Ubuntu (double-u?) for my next notebook. Also assuming Ubuntu runs well on it, which my guess is if it doesn't now, it will soon.
While I enjoy the fantastic gestures of OS X, I could survive without them. Netflix on the other hand, is irreplaceable. If (or hopefully when) Netflix runs on Linux, I'd consider it if the hardware were on par.
Titanium would be a good choice as well, but it is far more expensive.
Steel isn't a good choice for a variety of reasons, but #1 in my mind is surface finish. Typically you have to paint or plate steel.
A high end plastic or composite can be made nice, but people are moving away from it.
Besides the fact that this would make it not look like a MacBook?
>Something that would tell their product apart from a MacBook Air?
They're not aiming for "not MacBook". They're aiming for "we also have a MacBook!"