Ikari has just released the first version of 'freeBOOT' for xenon (v1) and falcon (v3) consoles with JTAG/SMC-Hack. This hasn't been tested yet, use at own risk.

Introduction
Finally, here it is - freeBOOT, a rebooter for the Microsoft Xbox 360. This version of freeBOOT allows you to reboot into kernel 2.0.8498 on a Xenon or Falcon console, which can be exploited by the JTAG hack. Support for Zephyr, Opus, and Jasper consoles will follow shortly.
As freeBOOT needs a second flash memory to store kernel 2.0.8498 and associated data, a Cygnos360 is required at the moment.
Read through the following instructions carefully. Building your own images is yet a complicated task involving many steps. Future versions of freeBOOT will be easier to build.

Power on your Xbox 360. If everything went correctly, you should see the blue LED light up a few seconds later, followed by the usual boot animation. If you power on your Xbox 360 with the DVD tray eject button, XeLL will be loaded instead.

Credits
Writing freeBOOT would not have been possible without the great work done by many hackers. Kudoz to arnezami, Redline99, Robinsod, SeventhSon, Tiros, tmbinc, xorloser, and anyone else I may have missed.

What's next
Features planned for the next release(s):
-support for all consoles
-update to the latest kernel

360-Hq Homebrew Database: Freeboot"