The way I understand it, is the arrest record cannot be removed. Even if you are not found guilty, the arrest will still show on a background check. I am in a similar situation in the state of Maryland. They have a similar program to ARD where the actual conviction will not be applied if I successfully complete probation. But I have been told the actual record of the arrest can be found if the the employer runs an extensive background check. Remember, there are many forms of background checks. You will have to do additional research for your state regarding expungement. In PA, you may be able to remove it after ARD. In Maryland, I can petition to have it removed after 10 yrs of clean driving.
Read the fine print carefully when completing the employment application. You may need to be prepared to provide documentation and an explanation of what occurred. Most employment application only request CONVICTION information, not arrest information, but if you honestly answer "no" to the conviction question (which I believe sucessfully completing ARD will prevent a conviction) and the employer finds the arrest, you will have to explain the situation. I was very fortunate. My employer was VERY understanding and even wrote a letter to the judge to verify my employment and job performance.
Being arrested for ANYTHING, especially DUI is horrible, but if you are truly remorseful and do the responsible thing to correct it, life will get better.
Good Luck.