As everybody by now should know, Apple released their update to their OS - Snow Leopard. For photographers, this update packs a lot of punch. According to these benchmarks, you get quite large speed increases in Lightroom (64 bit version especially), CS4, and Aperture, from going Snow Leopard and even bigger gains if you have a machine that can run the 64-bit kernel. So if you run Mac OS X and use programs like Lightroom or Aperture, this seems a no-brainer update. Even more exciting than the 64-bit part is the promise of even larger speedups with technologies that make it easy for developers to use the computing power on your graphics card (openCL) or trivial multiprocessing (Grand Central). So even bigger speed increases might be expected especially for image editing programs. Interesting times.
EDIT there is now a fantastic review of Leopard on Ars Technica. If you're a nerd like me there's lots to be excited about. The review is 23 pages and quite thorough.