I can't say I've taken a survey but Macs certainly don't seem conspicuously absent at a venue like FOSDEM (Brussels). Of course, that's an international event even if it's disproportionately European. It's probably fair that the further you get from US tech companies in general and possibly Silicon Valley in particular and the closer you get to developers doing open source in their spare time in non-US countries, the fewer Macs you probably see. If only because of price.

ADDED: And, of course, if you look around inside a company that mostly runs their infrastructure on Windows, most of the developers will be running Windows on Dell/HP/Lenovo/etc. machines.