Amid a bit of controversy, which can always be expected, Adobe announced that support for Windows XP will be dropped for Lightroom 4. This should not be surprising as Microsoft dropped mainstream XP support in April of 2009 and extended support will end in 2014. Typically, Microsoft ends Service Pack support two years after the official release of the product(or the end of the product life cycle if that is sooner). Support will continue for Windows XP for Photoshop CS6, but Vista will not be officially supported even though it will run on the OS. The user base for Photoshop is substantial compared to Lightroom which explains the continued support for CS6, but continued support is questionable.
Also gone is support for non-64 bit capable systems for Mac. Apple is no longer supporting 32-bit hardware with their current operating system, Mac OS X 10.7 (Lion), so if you are running an Intel based Mac system you will want to check to see that it is 64 bit. Mac Mountain Lion OS X 10.8 will be fully recognized by Photoshop CS6 as will Windows 8 OS. Most hardware will be supported by the new Mac system, but several graphics processors will not be covered. Mountain Lion is to be released this summer. Windows 8 is scheduled for release in the fall.
It is strongly recommended that users carefully refer to the operating system requirements of any software package they are considering to insure compatibility. While 64-bit systems are the way to go, there are still a lot of users out there with older machines and corporate systems that are limited to 32-bit. The required coding to support the older systems in the face of newly engineered advancements in the software make backwards compatibility expensive. Add to that the explosion of tablet computers and the job of making everyone happy just gets harder. Many people do not understand that 32-bit hardware and device drivers are simply not compatible with a 64-bit operating system. Many 32-bit programs will work, but not all. Newer programs and drivers written specifically to work in the 64-bit environment will not work at all on a 32-bit machine. This makes upgrading essentially imperative, especially for professional users.