A lot of airshow photographers see the 100-400 as virtually unassailable.
It has a lot going for it and if you don't mind push-pull zooms, it's a bit of a no-brainer. I usually use a combination of a 300 + 1.4 and a 70-200. But a 100-400 will give you at least as many options.
EOS 7D + grip, EOS 40D + grip, EOS 10D + grip
EF-S 10-22mm f:3.5-4.5, EF 17-40mm f:4L, EF-S 60mm f:2.8 Macro, EF 70-200mm f:2.8L IS, EF 300mm f:2.8L, EF 500mm f:4L IS, EF 1.4x Converter II
2x Speedlite 550EX