The boot ROM's recovery mode lets you load arbitrary code into RAM and then boot it. The conventional way to use this is load a flasher program that you then use to flash a new OS image, but you can use it to run any Linux kernel and and software of your choice purely from RAM, which can then do what it likes to the existing OS install. See http://linux-sunxi.org/FEL/USBBoot
> you can use it to run any Linux kernel and and software of your choice purely from RAM
As I said: sometimes you just want to run 1 app as root, not replace the kernel. Also, good luck getting a usable device with your own kernel/software when chip manufacturers don't provide drivers: unless you don't mind the usual litany of XDA caveats ("N0irROM v.0.4. Not working: Camera, GPS, Hardware acceleration. new: mic now working, randomly reboots less often")
