Yes, the rules (http://en.wikipedia.org/wiki/CoCom http://en.wikipedia.org/wiki/CoCom) dictate that the GPS should cut out if the device exceeds a speed of 1000

Yes, the rules (http://en.wikipedia.org/wiki/CoCom http://en.wikipedia.org/wiki/CoCom) dictate that the GPS should cut out if the device exceeds a speed of 1000kts AND an altitude 60,000 feet. A lot of GPS units wrongly interpret these limits as being 1000kts OR 60,000 ft, and so aren't suitable for high-altitude ballooning. Of course, it's a silly restriction because if you've got enough money to build a ballistic missile, you've probably got enough to build your own GPS receiver (CU Spaceflight have built a software GPS receiver themselves).