[Sorry for the self-reply; I couldn't fit this in the character limit:]
I want to give some background on my practice, in case it helps people who might want to comment (also, because it's neat to have a forum like this and feel like sharing, but let me know if I'm being too self-indulgent).
Today I had sat for an hour in the morning, but that session in particular I had been doing concentration (trying to get Jhana 1 better).
I found MCTB around January 5, and prior to that I had been meditating since about July 2007. My practice has been solitary and it has varied in diligence, but it had lately settled into 20 to 30 minutes in the morning and then again in the evening.
MCTB made a couple of things crystal clear: First, that I had been practicing mainly or only concentration all along, and second, that I had no idea what to look for when anapanasati-centered books talked about "contemplating impermanence," so I had been tuning out those parts thinking they were esoteric or bullshitty, or perhaps really advanced.
After MCTB I made a concerted effort to sense impermanence in the way it's described there and I was soon able to feel vibrations in my head, then with some effort in the breath. I've also been cranking up daily-life fast mindfulness and for the last week or so I've been hardly able *not* feel fast impermanence in many aspects of everyday experience. So there was a clear sense of build-up.
As I said I guess it's possible that this wasn't an A&P event, but it had all the earmarks. If this was indeed an A&P, it looks like, I have a dark night coming my way

Oops.