So are there any Express Card GPS receivers
available, or in the works that anyone knows of?
I don't know of any, but I do know that the positioning of these receivers can be quite important (e.g. putting it in the glove compartment may hinder reception of the satellite signals). Because of this, if the receiver is part of the expresscard, it would mean that the placement of the MBP is important. And that's not good because the MBP is much bigger than the receiver.
Another con of having an expresscard GPS receiver is that it doesn't make use of all that bandwidth. A serial port type of connection, with rates of 115kbps, is more than sufficient for the rate of data retrieved from the receiver. Using an expresscard would be a waste.
The best solution, to me at least, would be either via USB, or Bluetooth.