Took a bit of research - but I figured it out!

As of Feb 2009, "Gallery Plus" (the photo zoom feature in the search screens) is now FREE for certain categories - including Collectibles.

I only started using a Collectibles sub-category recently for a bunch of my stuff so I never noticed it before.

Here is the Ebay announcement:
http://pages.ebay.com/sell/update09/pictures/index.html

It's interesting that when I list in that category, the plus feature is automatically turned on. Nice!