Currently, Mojang only tracks when a player logs into the game. Notch's proposal is to also track "non-private data" like gameplay-session length, your PC's OS type, whether you're playing in single or multiplayer mode and whether you're playing via the client or the web applet. The collected information would then be shared with the community, "as usual."
"Naturally, the data sent will be fully anonymous," said Notch. "So it wouldn't contain any session information or your user name, and it wouldn't send any sensitive information that you might not want to share." To be perfectly honest, we're fairly surprised this kind of data wasn't already being collected. If you're part of the Minecraft nation, you can let your voice be heard by voting for or against anonymous data collection here.