Tony, I think that there is a fairly high number of users out there who would like to share their panoramas to Tumblr. Tumblr actually accepts photosynth.net embed code, so long as you use a video post. Example: http://natelawrence.tumblr.com/post/3007930593/
Here are some current sore points from the mobile scene:
1: Photosynth.net embeds work fine on laptops and desktops, but not on mobile browsers.
2: If I were on my iPhone or Windows Phone and had just uploaded a pano or spin movie to my Photosynth account, I would have to close the app, open Safari or Internet Explorer on my phone, sign into the website, click into the desired pano spin movie or synth, copy the embed code (assuming I already knew how to do this), and then paste it into the correct format of tumblr post. I think I can safely say that this is beyond most people's patience threshold.
3: I don't even know if you could copy the embed code from a mobile browser now, with the HTML viewer there.