Currently it lists 1,088 apps: 219 fit, 154 partly fit, 28 should fit, 27 are being updated, and 649 run letterboxed.
The initial data is collected from online sources, including Apple's announcement, release notes, developer's claim and newest iOS App builds fetched from App Store with majd/ipatool. And now owner's reports with screenshots are coming in. If you just got an iPhone Duo, you can submit a screenshot here too.