It's official - we've got some movement.
It's been faint and tentative for a few weeks now, but with certainty I can now say that the little buddy is making him or herself known when possible. The best part about it is it feels like s/he gets a running (swimming) start on one end and then just pounds into the other side. Clearly, this one has a high activity level (as does his big sister...mom and dad are going to be in trouble).
Jellybean still isn't big enough for this to be consistent or really apparent, but just feeling him/her there every now and then is a wonderful comfort. And another very significant milestone for us.