The application, called Conversations, is available for download from the Nokia Beta Labs Web site. While it is not a final release, even having a semi-workable beta application is better than nothing.
In order to get it to work, you have to take several steps. First, update your S60 3rd Edition E Series or N Series device's firmware to the latest version. Once that is complete, there are two separate downloads for the Conversations app that must be installed in a specific order. After they are installed, the application shows up as a shortcut in the application folder. It appears that the application runs independent of the actual messaging center, so if you want to view your conversations threaded, you have to open up the application.
According to initial user reviews and comments, it has some pros and cons. I would expect Nokia's developers will listen to all beta tester feedback and attempt to make fixes to the software for the full release.