Ground beef can be pink inside after it is safely cooked. The pink color can be due to a reaction between myoglobin (the pigment responsible for the red or pink color) in the meat and the oven heat. It can also occur when vegetables containing nitrites are cooked along with the meat. Doneness and safety cannot be judged by color, so it is very important to use a meat thermometer when cooking ground beef. To be sure all harmful bacteria are destroyed, cook all ground beef products to an internal temperature of 160° F throughout.

Additional Resources:

http://www.fsis.usda.gov/Fact_Sheets/Ground_Beef_and_Food_Safety/index.asp