I experienced this issue in my 2007, under the conditions described above. In addition to what's listed, I also noticed the traction control indicator on the dashboard flash. However, I did not experience the 'irritation' phase.
I was braking on an uneven surface, so I chalked it up to the TCS/ABS system taking a moment to adjust to what was going on. The first time it occurred it surprised me for a moment, but within a fraction of a second the brakes slowed the car. I had it happen several other times, always under the same conditions, and after the first time it no longer surprised me.
To be honest, it may or may not have happened in my 2010... it's such an unremarkable event that I don't even remember. When I first heard there were brake issues in the Prius I quickly checked it out, and as soon as I saw what they were referring to I thought "They've got to be kidding!" I've had several friends and coworkers ask if I knew about the problem. When I described it to one of them, she said "Oh, that? I've had it happen in my Four Runner."
If they have a recall / service event / whatever they end up calling it, I may not even have the work done, depending upon the details of what exactly the fix is going to do.