The iPod Touch is an example of something the general public has renamed as the iTouch. Perhaps Apple has taken this into consideration and suspects that if they do come out with a product titled "MacBook Slate" people will just call it the "iSlate."
Kind of like when people ask me if I have the Apple Air.
By your logic, Apple would've also reserved Air.com and Touch.com
I'm guessing the device's name was intended to be a word that has the zing to replace "book" or "magazine" or "tv" and can represent all of those functions. In that sense, iSlate works really well. It reminds me of the omnipotent monolith from 2001: A Space Odyssey.
iSlate is not a long word, nor an extension of any product family. It'd be the fourth leg in Apple's four-legged stool, as Steve Jobs would say.
-Mac
-iPhone
-iPod/iTunes
-iSlate
Each division with a different name and market in mind. Nice and simple.