A new version of XBReboot has been released already with a new patch (PIRS support) and fixed XeLL.
XBReboot is an image you can flash on your onboard NAND that will allow you to 'reboot' a JTAG-exploitable 360 in a patched 8955 kernel which makes homebrew XDK possible (unsigned XEXs etc).
The released image is just for Xenon motherboards, but the readme states it could easily be made for other 16mb-nand motherboards, so I guess we'll see support for those soon.
It comes with a readme this time, so we now get some official details: