Estimating real world object distance from photo I'm not really sure if this is possible but it should be.

I currently have a photo of a sphere, whose radius I've measures physically. Knowing the apparent size of the sphere from the photo and all internal camera parameters is it possible to estimate the distance from the camera to the sphere?

There is an equation, more used in lens imaging that is useful:

object_image_size = (real_object_size * lens_focal_length)/object distance

Problem is my object_image_size is in pixels and I need to convert it to a real world measure, preferable the size that actually showed up on the sensor.
Yes, theoretically, for the sphere no for other things in image. If it is possible to augment the situation then it is possible to measure anything you can track much more reliably. First, knowing the scale of a object that you can successfully track is a requirement to getting a scale for the image as the cameras lost all scaling info. But this requirement alone is not enough.

In practice you do not know the other settings of a camera (unless calibrated with stereo geometry first), all kinds of problems arise when you do real camera tracking. Such as real cameras lens not being ideal. By tracking more than 2 features on the scene you can automatically estimate the actual focal distances and camera distortions. More importantly you can get an error estimate on your data.

One way is to have two photos from different angles in a scene with enough depth variation (for a stationary object this is quite easy). You can then use stereo geometry to solve the depth many such tracker apps exist as part special effects packages and making your own is not entirely out of the question (fun link, fundamental matrix song). The second way is to have a known plane, the plane can be solved by knowing 4 points and their relative locations. In reality it is better use more points for error and lens correction estimates. A third way is to use structured light (a cloud of laser light points, or a line for example) to triangulate the point distances. Or you could use a light field camera.