I'm referring to the floating-point-esque numbers in parentheses that appear after the entity ID, at the end of the lines indicating that a player has logged in:
2011-04-21 23:49:37 [INFO] burpen [/192.168.1.2:45001] logged in with entity id 120884 at (26.5, 118.62000000476837, 72.5) 2011-04-21 23:49:45 [INFO] burpen [/192.168.1.2:45006] logged in with entity id 121098 at (25.5, 118.62000000476837, 83.5) 2011-04-21 23:49:53 [INFO] burpen [/192.168.1.2:45013] logged in with entity id 121638 at (20.5, 72.62000000476837, 79.5)
I believe these started appearing after the 1.5 server update, or one of the updates since then. I initially thought they denoted player location at time of login, but they don't match up with the values shown when I press F3 immediately after logging in. Moreover, the numbers change even when the player doesn't move (the three example lines were generated by standing in the same spot and repeatedly logging in/out).
I realize they might be part of some internal testing at Mojang that was left in for the most recent update(s), and in that case, they might not be terribly important. But if otherwise, I'm curious to see if anyone knows what these are.