In fact, I hope it has no HD at all. Even if it means limited capacity and high price, I want flash storage! Light, fast, power-efficient, and no moving parts! 64 GB would suit me fine. I have a 60 GB iPod to attach when I need more space.
I have a PB15 1.5ghz. It was top of the range, Apple's best, a mere 2 or so years ago. It only has an 80GB drive.
A subnotebook with 64GB flash would suit me fine for 90% of my computing needs. For the times when I need a large HDD, I'm happy to attach a small external. Ditto the optical drive.
Hell, aren't the 1.8'' drives up to 120GB now? Why not have one of these PLUS 32GB flash memory? The system with the most frequently used applications (MS office, firefox etc) and data could stay on the 32GB flash, and the 1.8'' drive could stay turned off, and only come on when needed.