Dexter Industries has just released its new GPS sensor - the dGPS. You can read all about it here.
I've had one for a short time and I have to say that it's a fun sensor to play around with... but 'play' may not be the best word - while the sensor can help your robots answer "Where am I?" it's also a sensor that can be put to use in the field... just attach it to your brick and you've got a handheld GPS receiver... use some of the built-in features such as calculating angle of direction, distance, and velocity, and you can use this sensor yourself for your own purposes.
For teachers and students wishing to learn how to use the dGPS and its NXT-G blocks (3 of them), I'm currently finishing up a workbook (100+ pages) with lots of hands-on activities, explanations, programs, and experiments to run. I'll release more information soon on when the workbook will be available (likely Dec 2010 or January 2011).
The dGPS is $105.00 US.