According to the official system requirements (http://helpx.adobe.com/x-productkb/policy-pricing/system-requirements- photoshop.html), 10.7 is not supported. While the 5.1 update came out after Lion did, this update was pretty minor and didn't change the system requirements. PSCS5 came out before Lion, which is why it's not "officially" supported. But for the most part I believe it works just fine. I'm using PSCS5 on 10.8 without issue.