The Google Places guidelines state that a "precise, accurate address" must be used to describe your business's location. You must use a physical location. PO boxes are not considered physical locations.
As of August 1, 2011 Google is going to begin deactivating places pages that use PO boxes as their physical location.
For businesses that use a PO box, they can add the PO box information into the second line of the address fields.
If you want to continue using Google Places, but don't want your address listed, Google suggests using the Service Area feature. This feature allows you to add your physical address location, but displays a service area, as opposed to displaying the actual location.
It is important to note, however, that the ranking of Google Places with service areas are sometimes lower than those Google Places that actually disply the physical address.
If you have any questions about Google Places - especially optimizing your place page - don't hesitate to post a comment or contact us directly!