I have been the agent provocateur on DOF issues as I have changed from the 350D to 5D.
In the course of many posts I have floated lots of bad ideas and as it turns out erroneous ideas.
First on DOF. It actually is much easier than I posited. Using the same lens, if one wants the same DOF using a FF camera as one has with a 1.6 crop factor, there really are two choices: frame the shot identically and stop down 1.3 stops (which will equalize the two) or pull back to the same distance from which one was shooting with the crop factor camera and shoot from that identical position (of course the subjects won't be framed identically but an after the fact crop can make them equal).
The related but hugely important issue is diffraction. Here we had some ideas floating that were not so accurate. Diffraction effects arise with small apertures. To simplify a bit, diffraction becomes a problem when the disk formed by a light beam spans across two pixels (give or take). Here is where the 5D then struts its stuff. Because its pixels are larger than the 1.6 crop sensor, it can be safely stopped down further than a 350D or 30D. Indeed it can achieve greater DOF because it can be stopped down more than it needs to equalize the effect of the focal length multiplier if the image is framed identically--ie its advantage is more than 1.3 f stops.
Here is a great tutorial link: Click Here For Tutorial