An XDA-Dev member has managed to mod the FireFireFire Extended Bootloader so that it can be used to dual-boot Amazon's popular Kindle Fire tablet.
As Brad Linder of Liliputing explains, normally FireFireFire allows modders to hold the power button for a few seconds during boot to load TWRP or another custom recovery app - instead of Android.
However, the modified version of FireFireFire lets users call up a boot menu and choose between "normal boot," "recovery," and "alternate boot."
Meaning, you could theoretically load CyanogenMod 7, CyanogenMod 9, or other custom recovery utilities. The source code for the multiboot project is available, along with a multiboot.zip file that can be flashed to the Kindle Fire using fastboot.
Nevertheless, as Linder cautions, Kindle Fire owners should probably refrain from flashing multiboot at this point in time, as the software (currently) only works if the 8GB of flash storage on the Kindle Fire is partitioned in a very specific way.
Still, the dual-boot option is a pretty sweet first step, which will probably make it possible to enjoy multiple operating systems on the Fire in the future.