One of the things that a BSD Unix admin learns to rely on is the "ports" collection, a cornucopia of packages that can be installed and managed by the particular BSD system's built-in package manager: pkgsrc for NetBSD, pkg_add for FreeBSD, and so on. In nearly all BSD systems, the port/package manager is part of the basic system (akin to APT under Debian Linux).
This provides the Mac user who has significant BSD experience with a nice, comfy security blanket. This blanket has a few stray threads, however. One of these is the package-management system and ports.
Software is customarily installed on Mac OS X systems from disk images, or .dmg files. When opened, these files are mounted into the OS X filesystem and appear as volumes, equivalent to "real" disks. The window that the Finder opens for that volume customarily contains an icon for the application to be installed and a shortcut to the Applications folder. Installation usually consists of merely dragging the application icon onto the shortcut (or into any other desired folder). Under the hood, things are slightly more complex, but two points should be borne in mind.
First, there is no true Grand Unified Software Manager in Mac OS that is comparable to APT under Debian Linux, or even the "Add or Remove Programs" item in Microsoft Windows' Control Panel. Uninstalling a program ordinarily consists of dragging the program icon to the Trash or running a program-specific uninstaller (usually found on the installation disk image).
Second, while there is a "ports" implementation for Mac OS X (MacPorts), it isn't a truly native part of the operating system. More seriously, the versions of software ports which are maintained in its ports list are not always the most recent version available from their respective maintainer. Installing an application via MacPorts, installing a newer version through the customary method, and attempting to use MacPorts to maintain the conflated software can quite easily introduce confusing disparities into the system, with potentially destabilizing effect.
Go back and read that last paragraph again, especially the final sentence. Mac OS X, meet BSD Unix. Touch gloves, return to your corners, and wait for the bell.
Most users will never run into any problems, simply because most Mac users make little or no use of MacPorts (or any other command-line-oriented system management tool). MacPorts users are (almost by definition) likely to be experienced Unix admins who pine for the centralized simplicity of their workhorse software-management system. Informal research suggests that many, if not most, MacPorts users are active developers of Unix and/or Mac software. In other words, we're all expected to be grown-ups capable of managing our own systems, trading away the soft, easy-to-use graphical installation for a wider variety of nuts-and-bolts-level packages.
So why is any of this a problem for me? Why am I consuming your precious time (and mine) blathering on about details which most interested people already know, and most who don't, probably aren't? As a bit of a public mea culpa and a warning to others to pay attention when mixing installation models.
I had previously installed version 8.2 of the PostgreSQL database server on my Mac via MacPorts. Returning to it later, I realized that the installation had not completely succeeded: the server was not automatically starting when I booted the system, and the Postres tools were not in my PATH. After a bit of Googling, I came across a few links recommending the PostgreSQL 8.3.6 disk images on the KyngChaos Wiki. The server failed to start as expected.
Remembering that I had previously installed 8.2 the "ports" way, I first uninstalled the newly-installed and -broken 8.3.6. I then ran port uninstall postgresql82 && port clean postgresql82 in an apparently successful attempt to clean up the preexisting mess, after which the KyngChaos disk images installed correctly and (thus far) work properly.
This once again points out the usefulness of keeping a personal system-management Wiki as a catchall for information like this — both to help diagnose future problems, and (especially) to avoid them altogether. These can be dirt simple; I use Dokuwiki, and have for over a year now. Forewarned (especially by yourself) is forearmed, after all.
Just thought I'd get this off my chest.