Sanding of the case and reseting the fuse with an electron microscope and an ion beam will of course allow rereading with an ISP. But I would consider this cheating
This was the sort of method I was thinking about, I recall the being another one, but I can't find it now
A for what the datsheet says, it also says
Atmel makes no representations or warranties with respect to the accuracy or completeness of the contents of this document
And even if atmel knew of a way to bypass their security, do you think they would go broadcasting it in their datasheets?
edit: ah ha, found it http://www.cl.cam.ac.uk/~sps32/mcu_lock.html
It appears certain clock and power glitches can cause a protected chip to become unprotected.