Is it true that the difficulty can be changed with a patch accepted by 50% of nodes? Doesn't this mean that an attacker only needs to set up a bunch of nodes and not actually have a majority of cpu power? How hard would it be to do an attack like that? What does it take to have a node? A 2880 bps modem and a Pentium 386? Can a computer hold multiple nodes?
Majority of the cpu power is probably more technically correct than 50% of the nodes.
As long as the patched nodes are generating blocks faster than the unpatched nodes they will 'win'.