When Apple unveiled its iMessage service in iOS 5 last fall, one of its perks is that it can sync your conversations between all your iOS devices. Now that Apple has released a public beta of Messages for Mac, those conversations sync your Mac as well.
In fact, if you’ve previously chatted with someone on your iPhone or iPad with an iMessage ID of your email address (not your iPhone’s phone number), 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.
Update: This feature might not be working right now, but it did this morning with the five or six people I tried it with. Also, be sure to use a person’s iMessage ID to make this work, not their AIM/Google Talk/Yahoo/Jabber. Just like in iOS, when you start typing someone’s name or address, you should see a blue talk bubble to the right of any address they’ve registered with iMessage.