I'm a great fan of micro 4/3 - I have an Olympus 4/3 and have owned the G1 and GH1. But, currently where they suffer for aviation photography is in continous focus. Tracking a moving object is much less reliable than with even a budget DSLR (though this is getting better with each new model).
On the plus side, their tiny lenses are great for shooting through fences, and with a 2x crop factor you get good reach.
I think if I were forced to choose between a DSLR and M4
/3 for aviation, I would go with a DSLR, though if you are shooting mostly statics or side on approaches, an M4
/3 will be fine.
Note that m4/3 has been developing very rapidly, so it would be worthwhile going for a newer model if possible.
Colin K. Work, Pixstel