It's no longer true.
Thanks to tmbinc, seventhson, robinsod, tiros... the new rule to fulfill is :
kernel<849x (summer 09 update) for model<jasper
or
MFR date<2009-06-18 for model=jasper (CB<6750 required)
All files and details are not known yet but it will involve some serious hardware modding
(resistors, diode, jtag adapter etc... wait for the details)
Even not knowing the cpu key you will be able to interfere with the booting process.
The result will be a 360 that instantly boots Linux without the need for patched King Kong.
Don't expect to be able to play games again on it though...
(it's recommended to have another 360 dedicated to games, because efuses etc... are still on the way, and future games will have mandatory update >=849x anyway)
EDIT :
Ok, tmbinc revealed that jtag hack has been fixed in recent Jasper models :
http://www.xboxhacke...p?topic=12241.0New condition is CB<6750, i.e Jasper MFR date<2009-06-18