When batch modifying XMP and IPTC header codes on JPEG files using the Adobe Photoshop File Browser, a new file is created with a new creation date and the original JPEG is deleted. Since Photoshop does not yet offer lossless JPEG rotation, I was worried that the new JPEG files with attached IPTC header codes may have been recompressed and undergone one step of JPEG degradation.
Luckily, my worries were unfounded, for a quick call to Adobe technical support revealed that IPTC header codes are appended to the file and no image data is changed. The fact that the original file gets deleted and a new file with a new creation date gets created is a known bug that has been logged and Adobe is looking into the problem.