If your browser opens an url, it sends its name (the browsers name), along with some versioning information.

What MSN is accused of doing, is looking at those names, and sending back a reply "Hey, you can't see this page because you're using Opera." Remember, when Opera-engineers changed Opera's ID to Opra, Microsoft -did- return the requested page.
This makes it look like Microsoft is filtering some browsers names, and denieng those names access to MSN.

If you would have to patch this, you would have to change the name you send to the server, you would have to change your products ID. Why would you do that? It's not even a bug.

Compare it to this: I have a fancy disco/pub/cinema, and you want to get in. I tell you, that because your name is Havenger, you can't get in. That makes no sence, does it?