Thanks - sounds like you found it. Since the last OSE update, I've been getting quite a few more links, all from relevant sites, but they're from small, low authority sites and a handful of big on-topic message boards (none are astroturfed- they're legit links).
That's what's so frustrating - as the content and backlinks were increasing, google traffic was as well - quite predictably, in fact. And it seemed like good legit traffic. Then bam - nothing.
Hopefully it will clean up soon.