There is a difference between DOF and "DOF". "Regular" DOF is "way in front of the lens", but the "DOF" you're thinking of is "right behind the lens".
When taking pics with a TLR we are not talking about 1:1 macro work, but about distances from 1m (3ft) to "infinity". Typically from a little more than 1m, especially with the Tele-Rollei. In this configuration the lens to film distance is roundabout 13,5cm or 135mm to 143mm to be more precise with the Tele (80mm Rolleiflex 2,8, 75mm Rolleiflex 3,5 and 55mm wide angle Rolleiflex) (the curved disks will give 8mm of travel to the lens board).
So, just a little movement of the lens towards or from the film will change the subject-distance in focus damatically. When checking the theory of DOF (like here
http://en.wikipedia.org/wiki/Depth_of_field or with online DOF calculators) you will usually find sketches / drawings that will not show proper scale. But with a "fancy" DOF calculator (sorry, I only have a German one at hand
http://www.mhohner.de/formulas.php#calc ) you can calculate not only the DOF regarding the subject but the lens to film distance as well. You will see to get 1mm change in lens to film distance (play around a bit!) you will have vary the lens to subject distance les with shorter lenses (well in case you have 35mm equipment including tele- and wide-lenses you can observe this directly).
http://www.mhohner.de/formulas.php#calc "neu berechnen" bottom left is the button to get the calculator calculate things over.
Brennweite (f) mm = focal length
Entfernung (d) m = subject distance (from film plane)
Blende = f-stop
Objektweite (g) 4.944 m = subject distance (from lens to subject)
Bildweite (b) 0.056 m = lens to film distance
Streuungskreis (c) mm = circle of confusion
Tiefenschärfe 3.913 m–6.922 m (3.009 m) = DOF from to
Hyperfokale Entfernung 18.01 m (9 m–∞) = hyperfocal distance
Format = format (I chose 6x6 from the drop down, but it doesn#t really matter
horizontal vertikal diagonal = (self explanatory)
Bildgröße (B) 56 mm 56 mm 79.2 mm = image size
Objektgröße (G) 4.978 m 4.978 m 7.04 m = covered subject size
Bildwinkel (α) 53.96° 53.96° 71.51° = angle of view
Abbildungsmaßstab (B/G) 1:88.9 (0.01×) = reproduction ratio