David Chartier:
>In fact, if you’ve previously chatted with someone on your iPhone or iPad with iMessage, then start talking to them in Messages on your Mac, the app will download your previous conversation and begin keeping it in sync with your other devices.
Not only that but I didn’t open my MacBook Air from 4p yesterday to 6a this morning and all of my iMessage conversations during that time when my Mac was asleep were up on my screen right away in Messages when I opened up my Air. This was at once very weird and also kinda cool.