I don't remember where I saw the tutorial but I remember that they used the warp tool a little and then putting a grayish box and using the gaussian (sp) blur to give it the shadow look.
It's easier is PS, at least for me, I'm not an AI expert. Just use the marque tool to draw the shadow around the box, fill it with a grey. Once you have the shape just use the blur tool to blur it a bit. Then you can adjust the transparency.
Its in steps 19 through 22 or 23 they do the effect your looking for. Its pretty simple as I mentioned earlier in photoshop you just add a drop shadow and use a blur on it and then use the warp tool a little to give it that lift.