The real answer is, to get the same image on a full frame sensor, you use a 45mm lens and crop 38% of the image off*. It's not like you are actually getting more focal length on a crop body, just less image.
And because you get less image - you have to stand further back to get the same framing with the same lens, so the depth of field at the same framing is larger on a crop sensor than a full frame one at equivalent apertures and focal lengths. I often find this confusing, but I guess if you are using a 45mm lens on a crop to fill the frame and a 72mm lens on a full frame to fill the frame you should get the same DOF as you will be the same distance from the subject... I've just basically repeated what you already knew haven't I?
In summary, what he said^
*Of course you may get fewer megapixels doing this depending on the megapixel count of the two cameras.