2.5" SATA, presumably. I don't know how many of the smallest notebooks accommodate that form factor, or have user-upgradable drives (for any reasonable definition thereof).
Have you looked at brands other than Dell?
Nope. The premise of the article is that comparable Windows notebooks cost about the same as or more than the MacBook Air -- I'm not planning to visit every PC manufacturer's Web site to find a much cheaper comparable machine that I don't believe exists.
“I’m not planning to visit every PC manufacturer’s Web site to find a much cheaper comparable machine that I don’t believe exists.”
Well I have to hand it to you, there is no way you will ever prove yourself wrong.
Pretty much all of them. It's simple to swap the drives on a Dell, HP, IBM, Sony, Acer, Asus, or most other PCs. A screw or two, out slides the HDD tray, a few screws and the standard 2.5" drive pops off. Add your new one and you're done. It's a standard size/configuration, and pretty much all laptops save the new Airs support that format and mount. It's nice to have standards for hardware!
I'm not planning to visit every PC manufacturer's Web site to find a much cheaper comparable machine that I don't believe exists.
No need to do so; the Dell 101z and the HP tm1z both are small laptops with equivalent features for about half the price (or a little less). Acer and Asus have equivalent models as well.