I see that there is a PPA for the test releases of Libre Office; this is not recommended for normal use.
The PPA for Ubuntu, as you know, tends to fall out of date. It is currently 3.5 on Precise (and unlikely to progress from there), whereas the latest release is 3.6.
I have hunted around, and cannot find a PPA for the latest stable release of Libre Office.
The only way that I can find to upgrade Libre Office it is to download the .deb and install, and keep an eye out for when it is updated.
Do you know where the PPA for the stable Libre Office release would be?