One repeated question is whether you can use PO Boxes for addresses in Google (or Yahoo or Bing or....) local listings pages (called Place Pages in Google).

The answer is a firm "maybe."

Address-Less Businesses

This is an issue I expect Google to resolve in the near future - it has been festering for quite a while, but with the huge emphasis on local search now, I have to assume it is on their radar screens to sort it out.