COMException trying to query the Global Catalog

I'm trying to query the GC from an ASP.NET application whose worker process

is running as Local System. It's throwing "...
InteropServices.COMException 
(0x8007200f): The directory service is unavailable." when it calls the 
FindAll() method. The code appears below.
 
//DirectoryEntry globalCatalog = new DirectoryEntry("GC://" + 
Forest.GetCurrentForest().Name);
//DirectoryEntry globalCatalog = new DirectoryEntry("GC:");
DirectoryEntry globalCatalog = new DirectoryEntry("GC://dc01.acme.net");
//globalCatalog.Username = "MyUser@[EMAIL PROTECTED]
";
//globalCatalog.Password = "Pas$w0rd";
IEnumerator gcEnumerator = globalCatalog.Children.GetEnumerator();
gcEnumerator.MoveNext();
globalCatalog = (DirectoryEntry)gcEnumerator.Current;
//globalCatalog.AuthenticationType |= AuthenticationTypes.ServerBind;
 
// Logic to define the filter and properties to load ...
string filter = ...;
string[] propertiesToLoad = new string[] { ... };

using (DirectorySearcher globalSearcher = new 
DirectorySearcher(globalCatalog)) {
    globalSearcher.Filter = string.Format(filterPattern, (gettingUsers ? 
"user" : "group"), filter);
    globalSearcher.PropertiesToLoad.AddRange(propertiesToLoad);
    foreach (SearchResult adPrincipal in globalSearcher.FindAll()) {
        // Do stuff with adPrincipal
}
 
I've left in several commented lines to illustrate some of the things I've

tried based on what others with similar problems have tried: connecting
with 
and without alternate credentials, serverless binding vs. binding to a
named 
server, binding with and without specifying a forest or domain. I'm not
sure 
that using the first child is necessary, but it doen't affect the outcome 
either way.
 
I can connect to servers in the forest using the API in 
System.DirectoryServices.AccountManagement, but the forest I'm working
with 
is spread all over the globe -- which makes things painfully slow -- and 
S.DS.AM doesn't give me a way to query the GC. Any thoughts on what's
wrong 
would be much appreciated.
 
TIA.
 

-- 
Rich Armstrong