Before I knew about cryptocoins, I ran a lot of GPU's for BOINC distrubuted computing projects. There are/were(?) heaps of CPU based Prime number finding projects but none of them ever used GPU, so I am agreeing that branching is not suitable for the hardware and that is why no one ever bothered to make a program to mine on them. For this reason I suspect we might never see a GPU or ASIC prime number miner.
This question comes from a complete non-computer engineer, but I'm curious. Since ASIC is designed to accomplish a specific task, wouldn't it be possible to create an ASIC to "mine" prime numbers? [This isn't a question about whether it would be a marketable device like the ASIC USB miners, just about the engineering side.]