Microsoft's removed the Photosynth apps for iOS and Windows Phone from their respective marketplaces, although the feature will live on in the cloud.
If you haven't grabbed the apps by now, you're plumb out of luck, as they've been pulled from their respective app stores entirely. If you do have the apps, you can continue to use them, but they are "officially unsupported and will not be maintained going forward."
This isn't the death of Microsoft's panorama technology, however, as the apps are being retired in favour of the company's Photosynth Preview cloud based service. Users of photosynth via app are, it's suggested, best off uploading their existing panoramas to Photosynth.Net.
Photosynth mobile apps are being retired [Photosynth]