I'm an experienced stock nature photographer making the late switch to digital. I have CS3 with the latest ACR, using a Canon EOS 40D.
When I choose images in Bridge to look at, they come up in ACR initially. I've played around with some of the simple adjustments in ACR, and when I exit, the changes show up in the Bridge thumbnails. In one case I cropped an image, carefully renamed it and saved it as a JPEG, and the crop shows up in the Bridge thumbnail. Same case yesterday - made various changes, exited without saving, and the Bridge thumbnail shows the changes. I'm assuming that the changes are recorded in a sidecar file, and that the CR2 file is unaltered. Is there a history tab I'm not seeing? Is it safe to work in ACR or should I go to Photoshop itself?
ALL changes made in ACR are metadata changes that are stored in the XMP file. The image data is never altered in the raw image, not in ACR and not in Photoshop. Unless you click on the Cancel button when you are finished editing in ACR, the changes that you make (including cropping) are stored in the metadata. So you can always open the raw image again and clear that crop and have the full image restored. Here is the absolute best book for Camera Raw.