I'm stool a little confused... but I'm glad the topic of conversation has gone back to full v crop and how we measure the difference... having said that... I'm still just as confused as when I first posed the question...
If I understand the question, see if this helps make sense of it:
Depth of field is affected by two things: absolute aperture and subject magnification. In terms of exposure, sensor size doesn't change anything, the whole "2.8 on crop is like 4.5 on FF" only refers to DoF.
If you took a picture, with a given aperture and subject distance, on FF and crop, you would get the same depth of field on both, just framed differently. (The FF image would have a wider angle of view.) if you framed them identically, your subject distance would have to change, therefore changing depth of field as well.
I just don't worry about it, as I'm quite happy with my 7d and the depth of field it gives me. :-)
Edited to steal PBD's words, since I had ignored focal length.