In what will certainly be controversial and disappointing to some Radeon Linux desktop users, AMD will soon announce that they will effectively be discontinuing support for several Radeon product families from their proprietary Catalyst driver. After that point, for future Linux distribution updates, the open-source Radeon Linux driver will be your only option for accelerated graphics. This is likely happening with the Windows Catalyst driver too, but at least there they have a better-maintained legacy driver process.
This summer, around the time of Catalyst 12.7, AMD will be dropping support for pre-Evergreen hardware from their proprietary graphics driver. This means that the Radeon HD 2000/3000/4000 series will cease to be supported by the mainline driver. The support will live on in a legacy branch of Catalyst, but that branch for Linux users will not be updated with new X.Org Server and Linux kernel support.
At the moment I don't know for certain whether the HD 2000/3000/4000 series support will be removed from the Windows Catalyst driver too, but it looks that way and I would assume so as the ~40-million-line Catalyst driver code-base is largely shared across platforms. The last time AMD dropped hardware support from their mainline Catalyst code-base, they did it at the same time for Windows and Linux. At least Windows users have the benefit of Microsoft maintaining a stable API/ABI for Windows so that the current driver will continue working for the foreseeable future. Even with AMD's older legacy Catalyst driver, they have made new releases to support Windows updates. On Linux, this isn't so easy.
The official announcement has yet to come out of AMD, but it has been confirmed by multiple contacts and is deemed important to get that information out there. Unlike the last time when AMD dropped several generations of hardware support from the Catalyst driver, there is still pre-HD5000 series being sold to this day. Soon Linux consumers will have no other choice but to use the open-source driver, which may or may not work for your needs, so it is important that this information is available now so you can make more informed buying decisions. In a quick look at a popular Internet shopping site still shows 25 graphics cards from the Radeon HD 4000 series still being for sale, 13 different cards for the Radeon HD 3000 series, and three graphics cards from the Radeon HD 2000 series (including the original R600; the Radeon HD 2900XT).
The last time there was a mass removal of hardware support from the Catalyst code-base was in early 2009 when they decided to drop R300 through R500 support.
With the Radeon HD 2000 (R600) through Radeon HD 4000 series (RV770) support going away, this means for all future major Linux distribution updates (Ubuntu 12.10, Fedora 18, etc), there is no option but to use the open-source Radeon Linux driver. With new X.Org Server releases and Linux kernel updates, the Catalyst driver before the support is discontinued from mainline (Catalyst ~12.6; fglrx 8.97 will be the last) will no longer be compatible. It is as simple as that. But...