To get a "acceptable" stale rate at 15s/block you pretty much need a local poold w/ longpolling + cgminer. (at 15 s/block and with poclbm+friends 5s askrate and no LP, you're looking at something like 20% stales)
That would explain things. Have the Bitcoin devs fixed the timewarp (sorry for lack of a better description, that's the only word I could think of) issue and what's there to stop someone from doing it right now? Or is there still a bit of muscle required to overcome the existing 10-12TH/s on the network?