DNN 3.1 was just released late in June. You can download it for free at http://dotnetnuke.com.
There are some differences you'll encounter bewteen DNN 3.0.12/13 and DNN 3.1.
The first thing you'll notice is there is no web.config file in the root folder when you unzip the files. Instead, there are two files named development.config and release.config. These are versions of web.config, and for your portal to work, you will have to modify the connection string in one of them and save it as web.config. If you're working in a development environment, modify development.config with your local connection string and save it as web.config.
Usersline module is now installed by default. You don't have to use the install procedure described under the Modules tab on this site.
Some custom modules that work with DNN 3.0.12/13 may not work with DNN 3.1. It appears that of the modules discussed on this site, that DotNetJedi counter, Spohn'software-Tabcounter, Orizonti NukeNews, DnnForge Where are they, and DnnForge Quotes modules work. DnnForge Quotes doesn't display the title properly on the admin page, but the rest seems to work. Orzonti Media Gallery 1.73 did not work on a test DNN3.1 installation. When I tried to configure the gallery I got an error. You may want to test custom modules before upgrading a production site.
*** I tried an upgrade on an existing DNN 3.0.13 portal on my dev machine and things crashed. That may be related to custom modules, not the portal itself.
***Remember that the documentation, such as it is, is found in the Documentation folder under the root. There is a readme.txt file there with useful information about some of the change.