[kstenerud]

Which part? The iCloud part or the OS install part?

I just finished reinstalling Yosemite an hour ago after wiping the HD of a friend's new laptop. It would not allow me to get past the eligibility check until I logged in with a valid Apple ID.

[X-Istence]

Your friend may have had Find my Mac turned on. At that point the only way to get an OS installed on it is to have the original apple ID it was turned on under.

[kstenerud]

Nope. She never used it. It was the initial "determining the eligibility of this machine" phase where it asked me to log in (originally, this was to check that you'd indeed purchased OSX Lion, but the OS is "free" now). If "Find my Mac" is turned on, it won't even let you wipe the hard drive.

[astrange]

Sounds like you were using Internet Recovery. It's downloading the OS installer from the App Store - if you had install media it'd work fine.

[kstenerud]

It's a macbook air. There is no install media.

[stephenr]

You can make one if you choose (most people use a USB stick, I'm using an ExpressCard SSD) - this will remove the need for an AppleID and internet access to do the install.

For the vast majority of people this will never be an issue as the machine comes pre-installed and re-installing the OS is generally not required unless something major goes wrong.