According to a spokesperson from 20th Century Fox, neither the studio nor the Franklin Mint meant to imply the U.S. Mint was involved, and certainly neither meant to break the law. But the marketing stunt may in fact be in violation, because it is illegal to turn federal money into an advertisement. This appears to be exactly what Fox and Franklin have done, since the 40,000 Silver Surfer coins are modified versions of actual U.S. quarters originally minted and circulated by the government in 2005.
It is hard to understand exactly how Fox and Franklin think the promotion, which advertises the website for the movie Fantastic Four: Rise of the Silver Surfer, did not imply the coins are real, legal tender. Though the Franklin Mint often produces collectible and commemorative coins, it does not usually distribute them for free by dumping them into circulation. The Silver Surfer quarters were sent into the currency stream last week and all are expected to be in circulation by the end of the Memorial Day weekend. If neither Fox nor Franklin means for them to be accepted as real money, then how are they to be distributed? So far neither company has been issued a fine or other legal ramification, but we'll see if something more happens with the story once the business week begins.
*Thanks to commenter Alex C. for tipping me off about the follow-up story.