Mojang's Notch has posted a poll to his blog asking the community if they mind the idea of the developer tracking anonymous and non-private data about the game from its players. This data would include things such as current game mode (single player/multiplayer), operating system (Windows/Mac), how long you've been playing a game session, or whether or not you play the game via client or the webpage counterpart.
Notch does want to ensure that all the data sent will be "fully anonymous" and it wouldn't contain any session information or username data. It is also information that would be shared with the community, "as usual." To voice your opinion on the poll for or against collecting anonymous data, be sure to check out Notch's blog.