Performing a clean install of Yosemite is not exactly straightforward. I first tried to perform a recovery disk install (after an upgrade) which failed. Next I tried to burn a USB installer drive using the common instructions that worked for Mavericks and this also failed with a "/Volumes/[X] is not a valid volume mount point." error. Luckily, I had a SuperDuper clone of my primary development machine and I was able to copy the installer app to this drive and boot to it on my now-bricked Macbook Air. Once booted I was able to successfully use the installer app to install Yosemite to the MBA's internal drive. If you're hell-bent on doing a clean install of Yosemite you may need a bootable external OS X disk with the 10.10 installer app on it to install from.
Update: My Macbook Air is doing fine but I've already heard from one person that a clean install in this fashion resulted in kernel panics on boot. It should go without saying but don't install beta software on a machine (iOS or Mac) you can't do without. Here be dragons.
Update 2: Twitter user Jaroneko posted instructions for making a bootable USB copy. I haven't had a chance to try it out yet though.
Update 3: Here are Jaroneko's instructions for burning a bootable USB drive. There was a line missing from the original paste but reportedly it works correctly now. Note that these commands assume the OS X installer app is in
~/Downloads and you have the USB drive mounted under