Thread safe queue (list) in .net

I need to create a thread safe list of items to be added to a lucene index.

Is the following thread safe?

public sealed class IndexQueue
{
    static readonly IndexQueue instance = new IndexQueue();
    private List<string> items = new List<string>();

    private IndexQueue() { }

    public static IndexQueue Instance {
        get { return instance; }
    }

    private object padlock = new object();

    public void AddItem(string item) {
        lock (padlock) {
            items.Add(item);
        }
    }
}

Is it necessary to lock even when getting items from the internal list?

The idea is that we will then have a separate task running to grab the items from indexqueue and add them to the lucene index.

Thanks Ben