Only for the moment! The PS3 OS is firmware based so it can be hacked. Exploits have already been discovered, but not utilised fully to gain access to unlocking the system. It is only a matter of time before someone does find a way.
Look at the PSP as a good example. To begin with, other than having the original 1.5 firmware people were stuck. Then suddenly exploits were being found nearly every week and the whole handheld console was unlocked completely over night to allow anything to be run.
And look at other systems. Dreamcast could run unsigned code without even being modded once the correctly patched discs were developed. Xbox eventually saw software modding after years of only having hardware mods available. And the first fully working software mods for the Wii now exist and work!
It will be hacked. It is only a matter of time. Anything that is physical hardware can be modified to bypass protection. And anything that is software based can be bypassed and hacked eventually.