I'm not shure if it is actually a bug.
I've tried to run Sabayon_Linux_DAILY_amd64_E17.iso from USB stick. I used Unetbootin for this purpose. Sabayon failed to get to runlevel 5 (no GUI in LIVE mode, no anaconda screen in GUI INSTALLATION mode). I've checked 2 versions: from March 25 and the previous one. I did it on 2 different machines.
Yes, I saw following doc on the wiki: HOWTO: Booting Sabayon LiveDVD Image from a USB Device with Grub. But:
1) the doc is almost 2 years old (it refers to grub (not grub2) and SL 5.2), so it is probably outdated
2) the same approach (unetbootin) works for Sabayon_Linux_8_amd64_Xfce.iso (which I was able to run in live mode and install on hard drive).
Maybe this is not a bug. But there should be some way to install SL via USB stick

