It's not really the GPUs that are the issue (once it's on the GPU I don't as far as I know see a way to hijack the system) it's the drivers that are the problem. Before your shaders can run on the GPU they have to be compiled and it's that that funny stuff can occur I believe. I guess you could maybe find a way to interfere with the other applications using the GPU for rendering.
Hopefully someone here as a good understanding of this stuff and can tell me if I understand the risk wrong.
once it's on the GPU I don't as far as I know see a way to hijack the system
I think GPUs can do DMA => If you can break into a GPU, you may be able to write into kernel space, just like you could (can?) break into a Mac over it's FireWire port (and, possibly, the Thunderbolt port in new MacBooks)
Hopefully someone here as a good understanding of this stuff and can tell me if I understand the risk wrong.