"All laptops will evolve to fit into a manila envelope."
I think this would be a good evolutionary target for laptops.
Yes, but making something thin doesn't necessarily make it more portable.
I don't consider anything larger than the iPhone to be portable: if it doesn't fit in my pocket, it's luggable, not portable. The test scenario is basically, if you're heading out for coffee on the weekend, will you take it with you? iPhone: yes, laptop: no.
That said, I spend a minimum of 90 minutes per day commuting in wifi enabled shuttles, so bringing a laptop with me is a requirement. The thinner (and thus lighter) the laptop, the less I have to lug around every day which is nice.