Vernon,
I don't know of a batch way to do it (Photoshop does batch conversion, but it won't handle this problem).
For the future, however, I have found that if I bring a jpg file into Photoshop for editing, and make any edits which produce layers then when it is saved it will be stamped with Photoshop as the creater application. The solution is to flatten the image in Photoshop before saving it. Select Layer==>Flatten Image to do this. The resulting file will not be stamped as a Photoshop file, and should open with Preview (assuming you have set that as the default standard for files with jpg as the extension). Of course, you give up the option to do editing on layers when you flatten the image.
If you decide to save a file using SAVE AS it also makes it a Photoshop file. To avoid this, make a copy of the file with the Finder to keep the current version and use Photoshop to save the original after you have made the edits. This avoids using SAVE AS.
There may be a better way around this problem. If so, I hope someone posts it.
