ok so i'm still none the wiser after reading a bunch of threads on this.
I'm UK, me and my dad have Boxster S's. Mine (2002, facelift) has power all of the time, my dads, a 2000, doesn't and only powers up with ignition.
so, maybe it is a facelift thing? maybe previous owners have changed it....?
either way... is there a way of stopping the power all of the time? i, like OP, don't much fancy the idea of the device plugged into the lighter (in my case, a BT/FM transmitter thing) draining the battery.