Yes, you could write an awesome encoder and have it offload some portions of the encoding task to the GPU for an appreciateable speed boost. [...] The x264 devs have discussed this. The problem is (from what they've said), that GPU programming is _hard_. Or, at least hard to get it anywhere near useful for x264, which has the vast majority of its processor-intensive code written in assembly
So basically, from what I understand, they've said "Patches welcome, it's too hard for too little gain for us at least for now."