This weekend I had a terrible (though obvious) realization: The Earth moves around the Sun at about 30 km/s. The disk velocity dispersion is also on the order of 30 km/s. Therefore, if you have a survey (like the SDSS Southern Stripe) that has a time span measured in a small number of years, the parallax will be detected at comparable significance to the proper motion. That is, you cannot measure proper motions without also simultaneously fitting parallaxes. Argh—and duh!