According to an article in the New York Times earlier this month, a review of 9 studies by the Cochrane Library showed some evidence that heat wrap therapy can be effective in relieving pain associated with short term (less than 3 months) low back pain. The review also found that exercise in conjunction with the application of heat further reduces pain and improves function.
The reviewers said that more studies would be necessary to really understand effects of both ice and heat on short term low back pain. If you are interested in the differences between ice and heat for back pain, well, there was conflicting evidence. (In this particular review, there was very limited information on cold therapy.)