Here's a really good article on the subject...
foodconsumer.org - Muscle Soreness -- Is Cold Water Immersion An Effective Treatment?
"After analyzing 17 trials involving over 360 people who either rested or immersed themselves in cold water after resistance training, cycling or running, researchers found the cold-water baths were much more effective in relieving sore muscles one to four days after exercise.
They concluded:
"There was some evidence that cold-water immersion reduces muscle soreness at 24, 48, 72 and even at 96 hours after exercise compared with 'passive' treatment."
Just how cold does the water need to be? In this case, most of the studies involved a water temperature of 10-15 degrees C (50-59 degrees F), which participants stayed in for about 24 minutes. Some of the trials involved colder temperatures or "contrast immersion," which means alternating between cold and warm water (this did not show a significant benefit compared to rest).
"