A beta version of Apple's new Messages app, which will be part of OS X Mountain Lion, is available to download now from Apple's website. If you're interested in giving it a shot, you'll need to have upgraded to 10.7.3, which rolled out with some difficulty a few weeks back. Some of us were able to download the app, though others have reported errors when visiting the Messages page on Apple's site. If you can get it to work, it's a quick 66MB download.
Of course, the big new feature (aside from the redesigned interface) is the integration of Apple's iMessage service. This previously was an iOS-only affair, but now you can send messages seamlessly between your Mac and iOS devices using your iCloud account. In a quick test, Messages on Lion seems to work much like it does when running on Mountain Lion. The install process was a little less elegant that we've come to expect from Apple and required a reboot, but was pretty straightforward in the end. Upon launching the app, you're prompted to enter your Apple ID (or create one if necessary); you can then choose whether you want to turn on read receipts and add additional email addresses you can be reached at.
Messages imports all your account settings from iChat, so if you've set up AIM, Gtalk, or other messaging accounts they'll be ready for you to use. Unfortunately, as we saw in our Mountain Lion preview, iMessages don't sync between your phone, iPad, and computer out of the box by default. If your friends are sending messages to your phone number (using iMessage as a replacement for SMS), they won't also automatically sync to the Messages app on your Mac — you'll need people to start sending messages to your Apple ID email instead. To make that easier, you can set your Caller ID to your Apple ID in the Messages app preferences on iOS. Once that's done, the service works beautifully, but it might be a bit of a pain to tell your friends to stop messaging you at your phone number.
Aside from these quirks (which already existed when trying to sync messages across an iPad and iPhone), Messages appears to run quite smoothly on Lion. If Apple can resolve the phone number / Apple ID issue between now and Mountain Lion's launch, we'll really have something useful on our hands.