About 2 and a half years ago, when we first started development on OpenLP 2.0, we had a look at all the technologies out there in order to decide what would be the best technologies to use in order to make a cross-platform version of OpenLP a reality. We settled on two in particular, a language called Python, and a graphical toolkit called Qt. In those days, the latest version of Python was 2.5 and the latest version of Qt was 4.4. So we made those versions our minimum software versions, and anything older than that we didn't try to program for, and we didn't offer support for.
Two and a half years later, we've looked at the current versions of software, and what minimum versions we should support. We looked at our supported operating systems, and their support, and decided to base our minimum versions off theirs. In particular, we had a look at Ubuntu Linux, which has two types of releases: short term support releases, and long term support releases (better known as LTS). With this in mind, we looked at the minimum versions of Python and Qt in Ubuntu's 10.04 release (the most recent LTS).
We aren't concerned with Windows or Mac OS X, since we have to build and compile OpenLP completely for those platforms, whereas the Linux and *BSD support OpenLP natively. Since Fedora, FreeBSD, and the other Linux and *BSD distributions are all up-to-date, and none of them offer the same LTS-style releases that Ubuntu does, we decided to use Ubuntu as our yardstick.
So, as of today, the minimum software requirements are Python 2.6, Qt 4.6 and PyQt 4.7, and the minimum version of Ubuntu is 10.04. The minimums for Windows is XP and OS X is 10.5. Fedora's minimum version is typically the release in development, and then all releases back to the current release. Arch Linux is a "rolling release" distro, and so OpenLP generally works on it. OpenLP generally works on all versions of FreeBSD as well, since it's a source-based system, and you can generally find the latest OpenLP package for PC-BSD as well.