Nope, even with a dipole antenna 100 km is possible. The main limiting factor for GSM is the ground and other obstructions.
Here's a back-of-the-envelope calculation:
Handheld power output of 2 W, dipole antenna of 1.5 dBi
Tower sensitivity of -102 dBm (that's actually the handheld, but the tower should be better), and dipole antenna
Work that out in free space with Frii's equation and you have a margin of 100 dB, which is 100 km. If you assume the tower antenna is pointed downwards and you therefore have a gain of, say, -20 dB, you've still got 10 km range.
Clouds and rain have minimal effect at 900 MHz, and the plane's clearly not that good of a Faraday cage, judging by the signal I get at the gate.
GPS manages to go 20,000 km with a measly 500 W equivalent (taking the antenna into account, in other words), as long as you have line of sight.