The S&P rating for Fannie Mae and Freddie Mac have been reduced from AAA to AA+, the reasoning behind this decision is long term debt as well as other subjective factors regarding the economy. This may have some effects on interest rates in the future, but probably not by much soon. It is hard to tell how this will affect future mortgage rates, but as rates today are already at a low, the increase may not be as detrimental. Currently the interest rates range from 4.5% - 5.5% for a 30 year mortgage.

Watch this video with NAR Chief Economist Lawrence Yun for more information

Sources:

http://www.washingtonpost.com/business/economy/sandp-to-assess-impact-of-us-debt-downgrade-on-local-and-state-govts-fannie-and-freddie/2011/08/08/gIQAJ72L2I_story.html