I can connect to XBL via the connection manager / test, with no reported problems, error codes, or warnings.
When I sign in to my account, I am immediately disconnected from XBL. My account is (newly minted) Gold.
Shutting off the system, plugging directly into the modem, and starting back up works fine. If this is considered a solution, then my Xbox is considered for sale.
I am using a Linksys WRT54GL. My system is an Arcade that I bought about...two years ago, I think?
I have port forwarding on AND the Xbox in the DMZ. UPnP is on. As I said, it connects to XBL fine. Until I sign in.
Some historical information:
I purchased a Kinect, which arrived yesterday. After months of well-deserved inactivity, I hooked my 360 back up. It was prompting me for an update, which didn't surprise me. What did surprise me was how resistant to installation the little *** actually was. After several hours of back and forth, trying to run network tests which dead-ended in saying I needed to update, and updates which said they couldn't connect, and I should run the network test, I finally found, buried on this site, something explaining that I could update via flash drive, which worked after only the second try.
Of course, there was another update (or two) for the Kinect, and it still wouldn't sign in to XBL until I connected it directly to the modem this morning. I was finally on XBL, and grabbed a few things from the marketplace, when I saw the 1 month gold for $1. I then made the most regrettable expenditure of $1 in my life.