Photoshop curved Shadow
I suspect that the curve on that shadow might be an optical illusion of sorts. As @lawndartcatcher explains in his answer, the curved look can be achieved by making the intensity (or opacity) of the shadow fall off towards either end.
Here is a step-by-step look at that process.
Here is my top layer:
Below that I add a basic soft shadow (I used a feathered selection to make it):
Now here's the part that gives the curved look. I screen this gradient over the shadow layer:
And I get this result:
Putting it all together gives something that I think matches closely with your reference:
Here is a look at my final layers in Photoshop CS3:
NOTE: I used a gradient with its blending mode set to screen to create the intensity falloff of the shadow. While this makes for a good visual demonstration, it really only works when you are dealing with a white background. To apply the same tenique to cases with different background colors, you would want to apply the gradient as a layer mask to the shadow layer.