The West Georgia Amateur Radio Society is in the very beginning stages of planning to launch a pico balloon. These are Mylar balloons (just like the silver party balloons) with an incredibly light payload transmitting its location using APRS or WSPR. These balloons are inflated just enough to reach the jet stream and then stall out there, allowing them to travel thousands of miles. Some have even traveled around the world.
WX4BK gave a presentation at our last club meeting about the WSPR mode that our balloon will be using. You can view it as a PDF here, and as a PowerPoint here.
We will keep you updated on the progress of our project and the information to track it once it's in flight.
Pico Balloon Payload (Image by WB8ELK)