Battlefield 3, but the Swedish devs only cursorily discussed it in a recent blog post. Most of what is shown off here was already gleaned from accidentally posted German screens earlier this month. DICE does point out one crucial aspect, however: "It's absolutely free."
It's still unclear whether the Battelog will be available in-game or solely accessed via PCs, but given Autolog's implementation in Need for Speed: Hot Pursuit, we imagine at least some form of the service will be available via consoles. Thankfully it seems that we won't be waiting long to hear more, as the post notes plans to reveal the "full suite of features" in Battlelog "at a later date." The multiplayer beta kicks off in just two months, so that "later date" will likely be "sooner" than ... erm .. well, you know.