I think even if all computers in Internet would install GPU,
we could crack something with about 80-bit key length,
but not more.
The old DES lies in this range. Triple DES perhaps at the limit. RSA with 1024 bit key length and Diffie-Helllmann are clearly outside, although RSA has some lacks that could be exploited (they use a 32-bit random generator and in principle GPU could generate all factors used in RSA).

Therefore a local network will not have enough power to crack a code.
However, what could happen is that cryptography could use a spread supercomputer to guaranteee more security.

I think the point is the following: a supercomputer over Internet is just a tool that can be used for doing well or doing bad. The choice is to the people who will use it.

Like a hammer: I can use it to build a nice house that will protect people from bad weather, or to smash it on the head of my neighbour.