Just checked the top 100, 200 and 300 BOINC computer systems. 

Top 100 - 6% are Linux systems 

Top 200 - 6.5% are Linux systems

Top 300 - 8% are Linux systems 

Top 400 - 7.25% are Linux systems

Top 500 - 7.6% are Linux systems. 

The Mac OS/X share appeared to be about half (or less) then the Linux share,
giving non-MS OSes about 10% of the top 500 comsputers. 

 
http://setiboinc.ssl.berkeley.edu/sah
top_hosts.php?sort_by=expavg_credit&offset=0