Imo a notebook is supposed to be small, and used in a minor way compared to a Desktop.
I installed a 40GB SSD into my wifes Dell Studio 1710, and she hasn't used much of it. If she decides she wants to download movies, I'll buy her a small form factor external USB 2.0 Hard Drive.
Imo 40-60GB SSDs should be standard on notebooks, with the option to add a 2nd HDD for media, or simply use an external for media.
Obviously there are some professionals that store large amounts of data on their netbooks, or simply have a "mobile office" in which case they could just order it with a 500 GB 2.5" internal drive.
My 2c