Yesterday, several Flickr engineers posted news of their layoffs from the Yahoo photo-sharing unit on Twitter, which caused GigaOm’s Om Malik to notice that Flickr architect Cal Henderson (pictured here) was also no longer on its About page.

According to several sources I spoke to, Henderson was actually not laid off at Yahoo (YHOO), but is leaving to start a new company–in the social-gaming arena, I am told–with Flickr co-founder Stewart Butterfield.

He and several other of the original core development team for Flickr transitioned out or have been transitioning out for quite some time, sources said.

Henderson’s Web site, called iamcal.com, still says he is the head of engineering at Flickr.

Butterfield (pictured here), as well as another co-founder, Caterina Fake, left Yahoo last year. Yahoo had paid $35 million to acquire Flickr in 2005.

Fake joined another start-up, called Hunch, last summer as chief product officer. Hunch is an answers site and is now in beta testing.