Use Windows Azure Blob to create a semaphore spanning the cloud.

BlobSemaphore.cs

// intended to be called only ONCE in real installations - or for a clean test run when no Jobs container exists
public static void GloballyInitializeJobManager(bool quiet)
{
   var blobContainer = AzureStorageAccess.GetBlobContainer(JobContainerName);
   bool didNotExistCreated = blobContainer.CreateIfNotExist();
   if (!quiet) System.Diagnostics.Debug.Assert(didNotExistCreated); // else, we probably should not be calling this method
   var blob = blobContainer.GetBlobReference(JobGlobalJobIdSequencePath);
   if (!blob.Exists())
   {
      blob.UploadText(JobGlobalJobIdSequenceStartingValue);

      blob.Properties.ContentType = "text/plain";
      blob.SetProperties();
   }
}

public static string GetUniqueJobId()
{
   // Nod to http://blog.smarx.com/posts/leasing-windows-azure-blobs-using-the-storage-client-library

   var blobContainer = AzureStorageAccess.GetBlobContainer(JobContainerName);
   bool didNotExistCreated = blobContainer.CreateIfNotExist();
   var blob = blobContainer.GetBlobReference(JobGlobalJobIdSequencePath);
   if (!blob.Exists())
   {
      blob.UploadText(JobGlobalJobIdSequenceStartingValue);
   }

   // TODO: ??? needs to handle case where it cannot initially Acquire Lease due to resource contention
   string leaseId = null;
   int retryCount = 0;
   var random = new Random((int)DateTime.Now.Ticks);
   var timeBeforeRetries = DateTime.UtcNow;

   while (leaseId == null && retryCount < 100 && (DateTime.UtcNow < timeBeforeRetries.AddMinutes(2)))
   {
      try
      {
         leaseId = blob.AcquireLease(); // lease is for 60 seconds and is renewable
      }
      catch (System.Net.WebException wex)
      {
         retryCount++;
         int sleepMilliseconds = random.Next(50, 1000); 
         System.Threading.Thread.Sleep(sleepMilliseconds);
         System.Diagnostics.Trace.WriteLine(String.Format(
                  "About to attempt retry {0} after sleeping {1} ms [exception status: {2}; exception message: {3}]",
                  retryCount, sleepMilliseconds, wex.Status, wex.Message));
      }
    }

    try { var leaseId2 = blob.AcquireLease(); }
    catch { }

    var blobContent = blob.DownloadText();
    // increment string to next one
    long jobId = Convert.ToInt64(blobContent);
    jobId++;
    string jobIdString = jobId.ToString();

    blob.UploadText(jobIdString, leaseId); // extension method
    // that writing new data to the blob turns this to application/octet-stream:
    blob.Properties.ContentType = "text/plain"; 
    blob.SetProperties(); // TODO: extension for passing leaseid 
    blob.ReleaseLease(leaseId);

    return jobIdString;
}