Hello diego,
I am also testing out DOF feature (for mayall) and a couple of things I found out are;
Not only the shutter speed and f-stop are important,  but also other points are important such as setting FOV (zoom), Focal distance, and scene size and placements of objects  also affect amount of DOF you get.
I am doing some test contains models in the foreground, mid and background spaced at more than 100 units (in cm), with;
 fstop = 0.5, 
 focal distance = 20,
 focal length = 35mm (FOV = 54.432)
 shutter speed = 50000 th of a second
objects placements are ;
 foreground obj = placed at 1 unit from a lense 
 mid obj = about 20 units
 background = more than 100 units
Maybe using MXCamera's center of interest checked and adjust by using Maya's camera's center of interest manip later would help find the focal point. ( Or you can also try camera's center of interest Unchecked and use the focal distance)
Try adjust FOV as well - narrower you get more of DOF, I think.
hope this helps,
-tak
					
										
					  															  					                

