I found a solution and did update the documentation for the GetXmlNodeByXPath method here:

http://our.umbraco.org/wiki/reference/umbracolibrary/getxmlnodebyxpath

But for some reason it has been rolled back to the original edit... I would try to roll it forwards but the history functionality dosn't seem to be working in chrome firefox or IE at the moment :/

Here is the code I used:

XPathNodeIterator otherAgencyAdmins = umbraco.library.GetXmlNodeByXPath("//Admin[@isDoc and agencyID = '" + agencyId + "']");

System.Collections.IEnumerator otherAgencyAdminsEnumerator = otherAgencyAdmins.GetEnumerator();
while (otherAgencyAdminsEnumerator.MoveNext())
{
    XPathNavigator CurrentNode = (XPathNavigator)otherAgencyAdminsEnumerator.Current;
    String strAdminRecordId = CurrentNode.GetAttribute("id", String.Empty);
    //do something with node id
}