The achilles heel of the XB360 is, as always, going to be software. At some point in the future, someone is going to ship a game with a buffer overflow. Hackers will then use this exploit to inject a loader onto the system in which to load their own shell. It's only a matter of time from there, until they figure out how to boot backup copies via this process. There may also be a flaw in the way the CPU based protections work that allows the PROM to be overridden.
Then there are more sophisticated hacks. If for example, the PROM checksum is based on a CRC or MD5, a replacement prom can be created that checksums to the same values. (MD5 has been broken recently)
Also, it may be possible to trick the CPU into encrypting a PROM for you with its internal key. In which case, the process of installing a modchip would be more complicated, but would involve first using an buffer overflow exploint to inject code which creates an encrypted PROM for you, next, storing that new PROM into EEPROM, and finally replacing the onboard PROM with your EEPROM.
I think it'll happen. Look how fast OSX was cracked, and they were also using a trusted computing module.