This reply of mine on
this thread may be helpful.
f/5.6 can give you a fairly shallow depth of field if you have the right zoom, the right distance between your subject and the right amount of separation between your subject and its background. On your DSLR, f/5.6 will give you a much more pronounced DOF than f/2.8 in a point and shoot camera, because of the size of the sensor. Full frame sensor DSLRs will give you optimal results.