Consider this, from the New York Times, which generally avoids the serial comma but routinely uses a comma after an adverbial prepositional phrase:

In the case of A.I.G., the virus exploded from a freewheeling little 377-person unit in London, and flourished in a climate of opulent pay, lax oversight and blind faith in financial risk models.

Why be sparing with the serial comma and generous with commas after introductory phrases? (And why use a comma after London?)

In February, A.I.G.'s auditors identified problems in the firm's swaps accounting.

The Economist is more sparing:

On September 18th it was appointed by Lehman Brothers' unsecured creditors to defend their interests.

With each passing day the news about China's tainted-milk scandal gets worse.

But with a long introductory phrase it rightly uses a comma:

On September 18th at the Elysee Palace in Paris, France's president, Nicolas Sarkozy, hosted Jin Jing, the handicapped Chinese Olympic torchbearer who had been accosted by pro-Tibet protesters in the French capital in April.

As Chris notes, that last example is a doozy of a sentence, with two appositives separated by commas in addition to the introductory prepositional phrase comma.

His rule of thumb for adding a comma to an introductory prepositional phrase is to pay attention to how the last word of the prepositional phrase could be read (or misread) with the next word. If the prepositional phrase ends in a word that could be perceived as a modifier of the next word, confusion will no doubt ensue.

Meta adds the following, from a Washington Post obituary on February 16 by Adam Bernstein for Steve Fossett (a court had finally declared the then still-missing Fossett officially dead):

In July, he was inducted into the National Aviation Hall of Fame.

In 2002, he traveled 18,827 miles

Over the next decade, he spent millions of dollars

Along the way, Mr. Fossett was credited with being the first

In 1998, a thunderstorm with hail ended his balloon flight

In 2005, he set an aviation record for solo, nonstop flight

She notes that any of these commas could have been spared without any loss of clarity. And changing usage (yes to serial commas, no to short introductory phrase commas) would likely save lots more space--the original reason behind the loss of the serial comma in newspapers and magazines. She would venture that introductory prepositional phrases appear far more frequently than series do.

Style is style, so there are no rights or wrongs, even with departures from style.

Our preference at ClearWriter is to be sparing in using commas after introductory prepositional phrases. We'll be collecting examples to see when using a comma helps readers and when not.

Future posts on introductory words, phrases, and clauses will deal with conjunctions, appositives, clauses, infinitive phrases, and more.

Source for examples: New York Times, Sunday, September 28, 2008, The Economist, September 27th, 2008, and The Washington Post, February 16th, 2008.