One situation where you might see immediate results is in High volume routing. I think this project [1]. They were using the GPU to saturate multiple 10GbE interfaces with a commodity processor.
I actually spoke to them about doing a similar thing, and they said they were using a GPU because they were pushing lots of sub-1500 MTU packets, and that commodity h/w could probably saturate multiple 10GbE interfaces that were just doing large packets for high-throughput.
1.http://shader.kaist.edu/packetshader/