I also forgot something. After I made the changes I posted above, I still had an issue.

If you have more than one product in a hidden category, visitors who are able to view one of your 'hidden' products will be able to use the previous/next buttons at the top of the product page to browse to your other hidden products w/in that same category. Depending on your situation, this may be undesirable.

Of course, you could edit the listing template, but this might be destructive if you wanted to show the buttons later on down the road. This is a non-destructive alternative using the CSS stylesheet.

Hide the Previous Next Buttons
A simple CSS rule will fix it. This is useful if you have multiple products in a hidden category that you don't want customers to be able to browse to easily.

NOTE: This will also hide those buttons in every category throughout your site (hidden or not), so use wisely. You might be able to narrow it down to only affect certain pages, but not very easily, I don't think.

Go to: stylesheet.css in includes/templates/css/ and add a rule to your css:
Code:
.navNextPrevWrapper { display: none; }
The rule will hide the whole section from prying eyes, but still allow you to make it visible later if you so desire.

Hope this isn't too off-topic.