A recent decision (Forest Group v. Bon Tool, 09-1044, 28 December 2009, precedential) from the Court of Appeals for the Federal Circuit (the appeals court that hears all patent cases, and sits just below the U.S. Supreme Court in the federal court hierarchy), clarified the civil penalty (up to $500 per article) for falsely marking a patent number on goods as applying to each article falsely marked.  

To put this decision in context, assume I manufacture a type of widget that is covered by my U.S. patent #1,234,567, and each widget I manufacturer is marked with that patent number.  Assume also that on 31 December 2009, that patent expired, yet in January 2010, I continued using my same tooling to manufacture and subsequently sell 10,000 widgets, each bearing the patent number.  Although the penalty for this false patent marking is at the discretion of the trial judge, I can be held liable for up to $5,000,000 ($500 per widget sold).  Ouch! 

On the other hand, if I fail to mark my widgets with that patent number while the patent is in force, I might not be able to recover damages from an infringer.

Given this tension in the marking requirements, caveat venditor, let the seller beware.  So you don't get caught in the traps, feel free to ask us about reviewing your products for proper patent marking.