How do I fade in an Object from half transparent to opaque in PowerPoint 2010?

The only way I found to do this was to add two copies of the object. That is, first add the object (say an image), then add another copy of the same on top. Set the top copy to be ~50% transparent (if it's an autoshape then via Fill / Transparency, or if it's an image then via Color / Washout). Finally, set the opaque copy to Fade In, and the semi-transparent copy to Fade Out at the same time. The effect should hopefully be what you're looking for:

(You can see the effect in action here.)

It is possible to do this by adding a new transparency animation and setting the transparency "Amount" to 0% in the effect options as shown below. The percentage here refers to "percent transparent", so 0% transparent would be opaque.

Since adding an additional transparency animation (to remove transparency) didn't work for me on PowerPoint 2010, here is my solution.

Create a white square (no outline) with transparency of the fill set to an appropriate amount to make the object underneath it appear how you wish, then make that white square "Appear" when you need the transparency, and "Disappear" when you want the transparency to be removed.