Based on idly counting laptops at conferences and other gatherings it seems plausible.  Yes some of those macs are dual booting Linux, but most are running OS X (based on highly unscientific and statistically weak observation) .  I won't dare to estimate numbers, but if you ignore developers working on operating system and desktop based projects, I wouldn't be surprised if OS X is moving towards becoming single most popular OS among the remaining open source developers.