The Japanese balloon bombs used in WWII were released from the Island of Honshu and were designed to travel 5,000 miles...they were found as far east as Michigan. I would imagine the materials used for the cell phone balloons would be somewhat more sophisticated and durable than those employed by the Japanese over 60 years ago...
Such baloon-carried explosives were the only (reported) casualties on the US mainland during WWII, I believe.
Family camping in the woods in California went and looked at one.