Just unboxed some new MBAs for my business users who were due for an upgrade, and plugged in my Mavericks bootable USB installer to them, but was greeted with a gray prohibitory sign. I also tried the Yosemite installer just to see if I could boot from anything and got the same thing. I know the drive works because I wiped another separate, older MBP earlier today with it and was successful. I have never had trouble alt-key booting into the USB drive before on the previous model MBAs. Has something changed with the latest and greatest MBA?