This is a hot topic in law right now because the law just changed. What you are describing is now handled under the statute called the Revocation of Paternity Act. Since the child is eight years old, mom would have one year to file a complaint from the date the new Act was put into place (June 12, 2012 ). First she would have to prove either that 1) she, the husband, and you at some time mutually and openly acknowledged a biological relationship between you and the child, or 2) the husband, having the ability to support or assist in supporting the child, has failed or neglected, without good cause, to provide regular and substantial support for the child for a period of 2 years or more before the filing of the action or, if a support order has been entered, has failed to substantially comply with the order for a period of 2 years or more before the filing of the action, or 3) the husband lives separately and apart from the child.
If mom can establish one of those things, and you deny you are the father, the court may order DNA testing. You aren't in much control of those things, but here is where your argument is strongest: the court can refuse to enter the order if it isn't in the child's best interest. The court can consider the age of the child, the relationship between the husband and the child, the relationship between you and the child, or any other appropriate facts. If she does file, I would strongly suggest you retain counsel because if you are found to be the legal father, you will be responsible for child support and any other court orders associated with the child until she is up to 19.5 years old. Best.
I agree with the the previous answer. Be aware, however, that either you or her husband can also file an action. It's best to consult with an attorney about your specific facts and circumstances to determine whether you should take this step. Any action has to be filed within one year of the new legislation.