I'm also hitting this error when trying to use the http protocol from the Flex API. My code is taken from the sample:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" applicationComplete="login()">
    <mx:Script>
        <![CDATA[
            import com.salesforce.Connection;
            import com.salesforce.objects.LoginRequest;
            import com.salesforce.AsyncResponder;
            import com.salesforce.events.ApexFaultEvent;
       
            private function login():void
            {
               
               
                var lr:LoginRequest = new LoginRequest();
                var conn:Connection = new Connection();
              
                    //set the username, password, token on component
                                       
                    lr.password = "myPassword" + "myToken";
                    lr.username = "myUserName";
                    var loginResponder:AsyncResponder = new AsyncResponder(loginCallback, faultHandler);
                    lr.callback = loginResponder;
                  
                    conn.protocol = "http";
                    conn.login(lr);
                }
               
       
            }
           
                       
            private function faultHandler(fault:Object):void
            {
                trace("fault " + fault.toString());
                if (fault instanceof ApexFaultEvent)
                {
                    trace("fault: " + ApexFaultEvent(fault).message);
                }
               
            }
            private function loginCallback(result:Object):void
            {
                  trace("logged in " + result.toString());
                     
            }
        ]]>
    </mx:Script>
</mx:Application>

This code logs in correctly when using the https protocol.  I get the following fault when changing the protocol to http:

I've also tried the suggestion at http://www.salesforce.com/us/developer/docs/api/Content/implementation_considerations.htm

to try the URL of:
http://www.salesforce.com/services/Soap/c/15.0 

conn.serverUrl = "http://www.salesforce.com/services/Soap/c/15.0";
conn.protocol = "http";

This also produces a similar fault:

fault [FaultEvent fault=[RPC Fault faultString="HTTP request error" faultCode="Server.Error.Request" faultDetail="Error: [IOErrorEvent type="ioError" bubbles=false cancelable=false eventPhase=2 text="Error #2032: Stream Error. URL: http://www.salesforce.com/services/Soap/c/15.0"]. URL: http://www.salesforce.com/services/Soap/c/15.0"] messageId="1600E819-CF4C-7C87-3989-63457DC378DB" type="fault" bubbles=false cancelable=true eventPhase=2]
fault: (mx.messaging.messages::ErrorMessage)#0
  body = (null)
  clientId = "DirectHTTPChannel0"
  correlationId = "000507BA-071A-A8AE-5521-634574DA02AC"
  destination = ""
  extendedData = (null)
  faultCode = "Server.Error.Request"
  faultDetail = "Error: [IOErrorEvent type="ioError" bubbles=false cancelable=false eventPhase=2 text="Error #2032: Stream Error. URL: http://www.salesforce.com/services/Soap/c/15.0"]. URL: http://www.salesforce.com/services/Soap/c/15.0"
  faultString = "HTTP request error"
  headers = (Object)#1
    DSStatusCode = 0
  messageId = "1600E819-CF4C-7C87-3989-63457DC378DB"
  rootCause = (flash.events::IOErrorEvent)#2
    bubbles = false
    cancelable = false
    currentTarget = (flash.net::URLLoader)#3
      bytesLoaded = 0
      bytesTotal = 0
      data = (null)
      dataFormat = "text"
    eventPhase = 2
    target = (flash.net::URLLoader)#3
    text = "Error #2032: Stream Error. URL: http://www.salesforce.com/services/Soap/c/15.0"
    type = "ioError"
  timestamp = 0
  timeToLive = 0
 

Finally I've tried setting the URL to "http://na5.salesforce.com"  and "http://www.salesforce.com", and in both these cases the response is a fault and is bringing back the HTML page source for those addresses.

I've run out of ideas at the moment and would greatly appreciate some suggestions and insight!

Thanks in advance,

David