Got the below from someone in the folding forums:
From: Zagen30
To: markfw
If you and your teammates were getting nothing but core 17 WUs until yesterday, I'm guessing you all were running client-type:beta. For the last few months this was handing out nothing but core 17's (probably not on purpose), but one PG member recently created a new core 15 project (7627) and sent it to beta. This has caused all of the 762x projects to have gotten a high beta priority and cut off that steady core 17 flow. On client-type:advanced and no flag, people have been getting mixtures of core 17 and core 15 work for a while. Each type seems to come in spurts, i.e. people will get several core 15s in a row followed by a few core 17's in a row, then back to core 15's, etc.
If you and your teammates haven't been using client-type:beta, then you must have just been extraordinarily lucky.
I'm PMing this because you're not allowed to discuss beta stuff on the official forums unless it's in the beta section, which requires one to be part of the beta team. I feel your pain, as I have 2 780s that have gotten nothing but core 15s for the past 3 days or so, and the PPD has dropped from 67-80% depending on which projects I get. The 762x's run my cards hotter than core 17's did and aren't as tolerant of OCing, so I've had to drop the clocks by about 50 MHz as well (1215 to 1175). I'm just sucking it up and hoping that they get more core 17 projects out, and accepting that the core 15 stuff needs to get done and that I've been avoiding them far longer than I should have been able.
As multiple people said in the thread, core 17 is the future, so we should all be getting the great PPD it brings on a regular basis in the near-ish future once the core 15 projects finish and more core 17 projects are generated.