What a week! First Sony has a little trouble with its online security, and now Microsoft is issuing warnings about people running "phishing" scams over Xbox Live.
A "phishing" attempt is when a player is contacted by another in an attempt to get hold of things like service passwords and credit card information. It's a fairly common thing over online services, but there are obviously enough attempts going on related to a single title at this specific time (in this case, Modern Warfare 2) that Microsoft needed to put up a warning about it.
It's hardly on the same scale as Sony's prolonged and widespread downtime, of course, and far from the first time such a warning has been issued for Xbox Live, but there are suspicions that the two are related, if only because all the attention focused on Sony's service has surely attracted extra scumbags hoping to cash in (and encouraged Microsoft to be a little more cautious than normal).
Microsoft's Warnings [Develop]