A couple of things I found out while trying to get this to work:
First, telemetry seems to work fine now with firmware version 2.4.8. I also found messages like this that seem to confirm it:
If anyone knows otherwise, I'd love to hear about it. :)
You'll need to know how the GPS is set up - which protocol it's using, and at what baud rate. The default value for GPS_PORT worked for me.
Also, I found out the hard way (by watching the AR.Drone flying full throttle into the ground) that you really shouldn't skip the magnetometer calibration step. This video walks through the various tedious but absolutely essential steps:
Another thing I had to do was to set the values for the local magnetic field in the airframe file. It's set for Toulouse, France by default. If that's not where you are, you should probably change it.
Will this get you up and flying? I'm not sure. I'm still waiting for my new GPS unit to arrive by slow boat from China, so I don't know yet. I found that trying takeoffs while holding the drone in my hand was a good idea, as it really wanted to roll hard right and crash when I had thing set up incorrectly. I was able to get the drone to take off and hover for a bit before I decided my GPS was too flaky to use for flying.