Updating already-deployed SharePoint content types to handle additional item events
Did you call ctype.Update(true) after adding the EventReceiver? If you don't it won't be persisted . And don't use the List content type, use SPWeb.ContentTypes instead.
This code works for me:
var docCt = web.ContentTypes[new SPContentTypeId("0x0101003A3AF5E5C6B4479191B58E78A333B28D")];
//while(docCt.EventReceivers.Count > 0)
// docCt.EventReceivers[docCt.EventReceivers.Count - 1].Delete();
docCt.EventReceivers.Add(SPEventReceiverType.ItemUpdated, "ASSEMBLYNAME, Version=1.0.0.0, Culture=neutral, PublicKeyToken=c5b857a999fb347e", "CLASSNAME");
docCt.Update(true);
The true parameter means it gets pushed down to all child ContentTypes as well. (i.e. to all lists using the content type).