Is it possible to render Unity graphics elements on top of iPhone / Android video feed?

it's really heavy for current devices. maybe later droid phones with great tegra 2 GPUs can do it or some of the windows phnes but currently it's not a possibility for applications that they are more complex than a moving sphere or cube. :)

The docs say that iPhone and Android can have plugins (not web browser though). If a plugin can be written to access the camera, they why not? It would be akin to the one that's already available (ARToolkit).

As to how well it would perform, that's caveat scriptor. If you want heavy duty graphics on top of video, good luck. But if it's something fairly modest, I think so.

So while that would not technically be what you are asking for (3d on top of video), conceivably you could get video into 3D (as a background texture or whatever).

Now we just need someone to take up the challenge and write those plugins.

See http://unity3d.com/support/documentation/Manual/Plugins.html