According to the Mirena website, it is OK to begin trying for a pregnancy as soon as the device is removed, (see quote below.)  At the Baby Center link you can read about the experiences of many women who were successful in becoming pregnant followind Mirena removal. Good luck, and let us know what happens!

http://www.mirena-us.com/mirena_right/convenient_birth_control.jsp

"If you decide you'd like to try to become pregnant, Mirena can be removed by a healthcare provider generally in one office visit. And you can start trying to become pregnant immediately. A woman's chances of getting pregnant within 12 months of having Mirena removed are approximately 80%."