Here's a good summation:
http://www.newalbanychiro.com/ice_vs_heat.htm
Which should I use? Ice vs. Moist Heat
Ice
Value: The use of cold (cryotherapy) in therapy is often applied to a new injury or a recently reinjured old injury to decrease pain, inflammation, muscle spasm, and swelling. Ice is the safest and most effective way of treating new or recently aggravated sprains, strains, muscle spasm and bruises. Health care practitioners frequently prescribe ice for a wide variety of muscle and joint complaints.
Sensation: When properly applied, the following sensations may occur during a normal treatment:
1. Cold
2. Pain or a burning sensation
3. Increasing discomfort or an aching feeling
4. Numbness or a significant pain reduction
The first time you apply ice it will usually be uncomfortable, but each treatment will get more comfortable.
Home Application: The easiest, safest, and most effective way of applying ice at home is to use a plastic bag half filled with ice cubes and wrapped in a single layer with a damp towel. The ice should be applied directly over the injury and left on for a period of 20 minutes. It can be reapplied as many times as needed as long as there is a 60-minute wait between applications. Generally, the application of ice to a new or recently aggravated injury is most effective in the first 24 to 72 hours.
Note: If you have circulatory complications or sensory losses talk to your doctor before you apply ice.
Moist heat
Value: Moist heat application helps relax tight muscles, and increases blood flow and nutrients to the injury which can help speed healing once the initial inflammation has subsided. It also helps the muscles and tendons respond better to being stretched and exercised. The treatment should feel like gentle warmth, never hot or burning. Moist heat penetrates deeper into the body and does not cause the injury to dehydrate like dry heat.
Home Application: When you use moist heat at home it should be applied to the injury for 20 minutes and then reapplied, as needed, as long as there is a 60-minute wait between applications. You can use a waterproof electric heating pad with a moist towel. Do not use a heating pad alone as this is dry heat and it may make your condition worse.
Note: Generally, never use any form of heat on a new or aggravated area within 2 to 3 days of reinjury. Although the heat will feel good at the time, it actually slows the healing process and may cause increased aching and pain shortly after treatment. For the first 2 to 3 days after an injury ice is the better choice.
Adapted from: Clinical Protocols, Thieme H., Lundgren L., Clinical Education Associates, 1996.