From CNET News.com

[..]
Norwegian browser maker Opera Software confirmed Thursday that it had noticed its browser is unable to access MSN.com. Several readers also alerted CNET News.com.

"Microsoft is actively keeping our browser from accessing MSN.com," said Jon S. von Tetzchner, Opera's chief executive. He added that every time a browser connects to a server, it sends a string to the server telling it what type of browser is seeking access.

"Microsoft is seeing (that) it is an Opera browser and shutting it out," said von Tetzchner, whose team was testing the problem. "If you change the Opera string by one letter, it is letting us in."

Microsoft admitted that its technology was watching for Opera strings–but only because the company wanted to encourage people to use standard-compliant browsers."

There's more, but that really takes the biscuit.

Someone asked Sir (hey, how long can it be?) Tim Berners-Lee for his comments:

[..[
Running the msn.com homepage through the [W3C] validator on Friday showed the site did not use valid XHTML and did not meet the Web Content Accessibility Guidelines.

So if the site doesn't use W3C standards, and browsers that provide more full support for them are blocked out, I would also like to know what the reason is.

Greed?