wunderground.com usually is more accurate than the other forecasts, but obviously it is too far ahead for any of them to be reasonably accurate yet. For the record, wunderground.com presently predicts:
"Partly cloudy. High of 52F. Winds from the NNW at 10 to 25 mph. Breezy."
In 2004 people were freaking out because the forecast said 75F (noon start). We ended up with 87F and all it did was make the spectators that much more drunk and crazy. Totally worth the 5-10 minutes most people lost due to heat.
I have been following the weather; I have a lot of friends running it, and they're freaking out about the weather. For a marathon starting in the late morning, the weather is of great importance. But last I looked, it's supposed to rain all day.