I was originally under the impression that dimension extension lines would automatically offset themselves from a centermark when the centermark's hole location was dimensioned. When this feature was introduced at SWW2009 it was received with great applause.
However, since installing SW2010, the behavior has been difficult to predict. Sometimes the dimensions will place with the correct spacing, sometimes it takes a rebuild and a few of them will gap but I always wind up manually adjusting the origin points of atleast a few of them before my detailing session is over.
I know that it only applies to drawing dimensions; not dimensions imported from the model. Beyond that I'm not sure what the rules for getting this to work are.