Okay, this part is only for those who have been using Camera Raw in previous versions of Photoshop (CS4, CS3, and so on), because if this is the first time you’ll be using it, this won’t affect you at all, so you can skip this. Here’s why: in Photoshop CS5, Adobe dramatically improved the math behind how it processes noise reduction, sharpening, and post-crop vignetting for RAW images. If you have RAW images you edited in earlier versions of Camera Raw, and you open them in CS5, you’ll have a choice to make (though, I think it’s an easy one).
When you open a RAW image in CS5’s Camera Raw 6 that you previously edited in Camera Raw from an earlier version of Photoshop (like CS4 or CS3), you’ll see a warning appear in the bottom-right corner of the Preview area (actually, it’s an exclamation point, shown circled here in red). That’s letting you know that your image is still being processed using the old Camera Raw processing algorithm from back in 2003, but you have the option of updating the image to use the new, improved processing, called “Process Version 2010.”
To update your previously edited RAW photo to Process Version 2010, you can either click directly on the exclamation point warning (which is the fastest, easiest way), or click on the Camera Calibration icon (it’s the third icon from the right at the top of the Panel area) and choose 2010 (Current) from the Process pop-up menu at the top of the panel (I’d only do it this way if I was charging by the hour). Now, if your image didn’t have any sharpening applied, or noise reduction, or post-crop vignetting, you’re not going to notice a change, but if it did, you’ll be amazed at how much better it looks now.