According to a news post that recently showed up on gaming news site N4G, Microsoft has banned over 600,000 user accounts in the last few months and aims to cross one million bans before the holiday season is over.
Before you start sweating bullets and running to your console to check if you have been mysteriously banned, you should know that these bans are specifically targetted toward people who have modified their consoles so that they can illegally play pirated games. From time to time Microsoft cracks down on these activities with "ban waves." Whenever one is initiated, it only improves gaming for the rest of us by diluting the population of jerks online. If you buy all your stuff the normal, legal way then you will be fine.
This news is still only a rumor anyway as the article states that the information was obtained from a "friend" that works in a call center for the Xbox 360. Not exactly a rock solid reliable source. Nevertheless, if you have modifier your console in any way, you should beware (and be ashamed of yourself).