Engadget writes:

"The obvious answer as to why Xbox Live has been up and down (but mostly down) over the last couple of weeks is, of course, that the service has undergone a massive distributed denial of service attack. (Feel free to blame the illusory Sony / Nintendo fanboys, if you must.) Naturally, we"re already starting to see some of those rumors catch hold, in no small part due to the fact that while GM of Xbox Live Marc Whitten did offer a free game for peoples" trouble in his public memo a few hours ago, he didn"t speak to the cause of the outages"