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My best guess is that it's used to identify the type of power supply (65W vs 90W) that's connected to the system. Given the adapter that's described in this thread: viewtopic.php?f=18&t=91992
, not connecting anything to that center pin might mean that the laptop will assume that the 65W supply is being used. I say that since to me that's the logical choice; you wouldn't want the system to think it's using a 90W supply and configure itself to use more power than what's available from the adapter.
My X60 systems report correctly that a 65W supply is connected (under the Battery tab of the ThinkVantage Power Manager). I don't seem to have a 90W supply around here but I'd imagine that if one were used, the Power Manager would "see" it. I've measured the center pin with respect to ground; the metal on the outside of the DC plug. There's no voltage present when the supply is plugged in but there is a resistance of 10Kohms when it's unplugged. Maybe if somebody has a 90W supply they can measure the resistance and report.