There are at least two groups that know of that launch High altitude balloons with amateur payloads. I just joined one: Arizona Near Space Research. http://www.ansr.org/
The other s in Colorado, not sure of the organization's name though.
Edge of Space Sciences. http://www.eoss.org/
The balloons typically reach 90-100K feet in altitude and carry a Crossband repeater and several other payloads, including APRS Digipeaters & SSTV. The footprint is about 300 Miles at burst altitude.
Actually, the footprint has a diameter of several hundred miles. For the ANSR launches from locations in central Arizona, the footprints easily cover almost all of Arizona along with portions of several neighboring states and northwestern Mexico. The following link is a bit old, but the coverage maps at different altitudes are still good and show this better than a text description:
Patrick WD9EWK/VA7EWK http://www.wd9ewk.net/