Для следующей бедной присоски, работающей без документации:
/// <summary>
/// The SharePoint Web Service: Lists.
/// </summary>
private readonly sharepoint.lists.Lists wsLists = new sharepoint.lists.Lists();
private string GetListUrlFromName(string listName)
{
XmlNode node = wsLists.GetList(listName);
return node.Attributes["RootFolder"].Value;
}