In the latest update to ACR (7.1 or Lightroom 4.1) there is a new tab you need to pay attention to. The tab is labeled “Color” but the correction is for chromatic aberrations, and specifically addresses issues with what is known as purple fringing.
Both the Profile and Color options are off by default, but I suggest that enabling them is a good idea and simply eliminates having to remember to check the settings every time you edit an image. The image at the left is from ACR 7.1 in CS6. Lightroom 4.1 has a similar “button” in the Lens Corrections section of the Develop module.
Remember that you can “Save New Camera Raw Defaults” to make this happen automatically. I suggest opening a raw file and changing nothing in the normal adjustment panel, but making whatever other modifications you would like to see happen automatically throughout the raw dialogs and then saving them as the default. In the Raw Preferences, you can choose to have these modifications specific to different camera serial numbers and even to ISO settings.
Purple fringing is different than typical red/green and blue/yellow fringing which has been handled pretty well by previous versions of the raw converter. Purple fringing and the companion green fringe on the opposite side are usually evident in areas in front of and behind the plane of focus and are not necessarily a problem near the edges as are other fringing issues. They can have an effect on images with small neutral areas, high contrast edges, and places where there is flare. They are more prominent in fast lenses.
The ability to handle this particular issue is a big step forward in producing clean images from raw capture. Simply checking the “Remove Chromatic Aberration” box should work well enough in most cases, but there are Defringe controls below to handle stubborn cases. The good news is that you will rarely need to use them. For more detailed information on that point you may want to read this: