The optimal temperature is 44°F.
50°F source: Dr. John Cantwell, chief medical officer, Atlanta Summer Olympic Games
54°F source: American College of Sports Medicine
55°F source: "Physiology of Sports", by Thomas Reilly
"...air temperature of approximately 10°C (50°F) is optimal for marathon running" source: "Impact of Weather on Marathon-Running Performance" published by Medicine & Science in Sports & Exercise
50 to 55°F source: Dr. David Martin, professor of physiology, Georgia State University
"The ideal marathon temperature, according to these analyses, was a bone-chilling 41 degrees." - Source: U.S. Army Research Institute of Environmental Medicine.
Dueling studies aside, the bottom line is that anything below 40 degrees is too cold, anything over 55 degrees is too warm, and temperatures in between are very conducive to good marathon performance.