35 U.S.C. 287 generally requires tangible products to be marked with the patent number or else no liability for infringement exists until the infringer is on actual notice.  Section 287 provides in part:

(a)  Patentees, and persons making, offering for sale, or selling within the United States any patented article for or under them, or importing any patented article into the United States, may give notice to the public that the same is patented, either by fixing thereon the word “patent” or the abbreviation “pat.”, together with the number of the patent, or when, from the character of the article, this cannot be done, by fixing to it, or to the package wherein one or more of them is contained, a label containing a like notice. In the event of failure so to mark, no damages shall be recovered by the patentee in any action for infringement, except on proof that the infringer was notified of the infringement and continued to infringe thereafter, in which event damages may be recovered only for infringement occurring after such notice. Filing of an action for infringement shall constitute such notice.

There are provisions for process patents (on which you cannot place a patent number).  For more information see this article.

35 U.S.C. 287

On September 16, 2011, the America Invents Act (AIA) revised a civil action that could be brought for falsely marking a product as patented when, in fact, it was not.  Prior to the passage of the Act, so called “patent marking trolls” brought actions against companies that frequently keep expired patents marked on their products.  The civil action awards $500 per false marking which is split 50/50 with the government.  The AIA amendment now requires the entity bringing the false marking action to be competitively injured as noted by the Federal Circuit in its May 2012 decision.

The twenty (20) year term of a patent begins to expire as of the filing date of the patent application. Therefore, the inventor or applicant should begin marketing efforts as soon as the application is filed, i.e., the “patent pending” period of time should not be wasted.

The filing date and the application number should be treated as trade secrets, i.e., neither the filing date of the application nor the application number should be published. This prevents a potential copier from making an educated guess as to when the patent may issue. No copier will go through the time and expense of preparing to copy a product when the issue date of the patent is unknown and unpredictable.