While I'm on a Win system, there should be something on the Mac OS that lets you assign what program will open a particular file type. With Win, you just right click on the file and select "Open with" then select use as default. Should be something along those lines on the Mac also.
O.k., so far so good. Now, the question is; will it work in Photoshop for smart objects?
Well, no that does not work in Photoshop. Yes, on the desktop, right clicking on an icon gives you the option to open the file in the application of your choice. However, in Photoshop right clicking on a smart object icon gives you a different set of options and choosing an application to open it is not one of them...
So, still waiting for another solution...
If anyone else has this issue, I found the solution in Apple Forums.
If you have a Photoshop file that has a Smart Object which defaults to Preview, get out of the file. At the Photoshop file icon, right click it (or control click if you are still clinging to that old single clicker) and choose Get Info. In this file, towards the bottom of the page there is a button called Open With - choose Photoshop. Oh! Also, before closing out, be sure to click on another button just below this that says Always Open With (or something to that effect). Close your Get Info window and open your Photoshop file. Now, when you double click the Smart Object icon it should open the Smart Object in Illustrator.
Yeah, sounds like somehow the OS had .AI files associated with Preview instead of Illustrator.
Correct the file association, and things should be happy again.