The only thing close would be a large-screen cellphone with internet
access. But most people report that these will not acquire adequate
signal strength about 3000 ft agl or so. The old Palm System was
wonderful in comparison.

Another possibility might be a satellite "cellphone" system with
internet access - but I'm guessing here.

The cheapest PDA route probably runs $2000 or so including the cost of
the PDA.