Adobe yesterday has admitted a bug in the recent RAW engine for Photoshop (Adobe Camera RAW 5.5) and Lightroom 2.5 for Mac users with a PowerPC and with specific cameras (Pentax, Olympus, Panasonic, Sony and some medium format).
This bug produces artifacts in highlights and has been introduced with the demosaic tuning in the last release. While Adobe is preparing the fix, affected users should downgrade to previous version.
Official announcement availabe at this link.