Could the pioneer anomaly be a small scale example of the effect that causes higher-than-expected angular velocities of stars in the outer regions of galaxies (dark matter, if you wish)? Both seem to suggest an unnacounted gravitational attraction to the center of the system in question. Are the unaccounted forces proportional?
Also, the wiki article on the anomoly states that the additional acceleration is very close to the product of the speed of light and the hubble constant. Is this significant?