Can a mother take kids out of state without letting the father know the address of where they are staying?

I and my wife are separated (not legally) and I am filing paperwork for divorce this week. My 2 kids have been living with me full time since we went our separate ways in October 2010. She had moved to Utah a few months ago and recently picked up the kids for a month long visit that we agreed upon. Once she picked up the kids she called me and told me she is not bringing them back. I have full contact with the kids via their cell phone. But she will not give me her address, even when I ask. There are no custody papers that show who gets the kids yet and she has her right to the kids. But can she take them and not give me the address where they are staying? Also, Like I said I am filing for divorce and I need her address so she can get served. How do I go about getting her served without it or does she have to give me her address so I know where the kids are?