Fuutgut,
The Position tolerances on the datum features still have meaning. As pmarc mentioned, datums are perfect but datum features are not. Datum features are imperfect as-produced surfaces on the part, and datums are theoretically exact planes, lines, and points that are extracted from datum features. This is one of the main concepts of the datum reference frame section of the Y14.5 standard.
In your sketch example, the Position tolerance on the slot controls the slot's centerplane within a tolerance zone that is two parallel planes 0.5 mm apart. The tolerance zone is exactly perpendicular to datum plane A, and exactly in line (centered) with datum axis B. So the Position tolerance controls the slot's centerplane to be perpendicular to datum A within 0.5 mm and centered on datum B within 0.5 mm. The Position tolerance does not control the form of the slot - the size tolerance would do this (because of Rule #1).
The datum extracted from the slot would be a theoretically exact (i.e. perfect) plane that is, by definition, exactly perpendicular to datum A and exactly in line with datum B.
When a feature is used as a datum feature, one could say that certain aspects of the feature become perfect. For datum feature A, the height (say, the Z coordinate) of the high points becomes perfect because we're creating a reference frame that is "zeroed" on a plane that goes through these high points. This plane becomes the origin for measurements in the Z direction. If you created the ABC alignment on the CMM and then went back and measured points on datum feature A in that alignment, the Z coordinate of the high points should read zero (i.e. perfect).
By the same token, the clocking rotation of datum feature C (strictly speaking, its related actual mating envelope) should be perfect in the ABC alignment. The orientation to datum A and the centering on datum B will not be perfect, but the clocking of the AME will. This is because the clocking rotation is the degree of freedom that is constrained by datum feature C.
Sorry about all the wordy text descriptions - if I had time to make a couple of sketches it would be much more clear.
Evan Janeshewski
Axymetrix Quality Engineering Inc.