My guess is, the gps actually uses the DTR line to detect whether the serial port is active, from what I found on the net that would be pin 1 i.e. the opposite side of where the ground is connecting to. But it also activates similar to how the empeg detects whether it is on AC or DC, probably by breaking a ground line when the plug is inserted which makes the 'dtr' line float high with a pullup resistor. Maybe it also turns on when it sees data on the RX line. but I highly doubt that. It is more likely that with an unconnected DTR the line 'naturally' floats high, implicitly activating the receiver. But then again even that would be suspicious because that would activate the receiver whenever it is disconnected from the palm.

Perhaps the best thing to do is to connect DTR, and possibly wire it directly to the power lines on the board as it will provide a nice and regulated source without negative spikes and probably fewer dropouts. It also allows the receiver to go to standby whenever the empeg powers down or is removed.