I have Windows 8 and Ubuntu 13.04 dual booting with UEFI and grub. (re-used the windows UEFI partition) After I used boot-repair, everything seemed fine. After I boot into Windows, however, I can't boot into Ubuntu anymore. Grub still allows me to select it, but the screen just goes blank and there are no error messages.
Here's the really weird part: If I have the liveUSB stick plugged in (but not being booted from), then Ubuntu boots. And after Ubuntu boots, I can remove the live USB stick and continue to boot normally into Ubuntu... until I boot into Windows 8. Then I need to have the live USB key to "unlock" the ability to boot into linux again.
I've heard of Dell's software writing to the EFI partition, but this is an Asus machine and I haven't heard of their shovelware doing that.