A playgroup friend we see occasionally commented on this the other day, remarking, "She's not the same child!" Well, yes and no. She is who she's always been, and she's becoming who she will be. Just like her seemingly-continual physical growth spurts, her communication/social growth is happening all the time. I think now we're just paying closer attention.
I must say I am quite pleased to see her interacting with other kids more in a mostly-appropriate way. She has started sharing toys more frequently with less fuss, and rather than avoiding playmates entirely (or screaming when they come near), she will observe them and take part in lateral play - even if she is still a bit of a bulldozer, now she is learning "gentle with friends", so is pushing gently with one hand rather than a full-on body check!
This progress does seem to have one catch; the better her communication gets, the more frustrated she is getting when she can't make herself understood. Tantrums are abounding lately. Then again, she's also 21 months now, so "terrible twos" may be imminent. I'm really hoping the OT and PC will help with some strategies for that in the fall.