O/S GL Version Installs
Windows 98 1.0 28
Mac OS X 1.1 10795
Mac OS X 1.2 4186
Windows XP 1.3 25154
Windows XP 1.4 10584
Mac OS X 1.5 30518
Windows XP 2.0 11256
Windows Vista 2.1 181
Taken from 1 Jan 2007 till today. Note OSX is so far behind XP. Still 25% of OSX machines are using GL 1.1. No 2.0 support in OSX yet.
Cas 
Looks like 1.1 as a minimum spec is still worth targeting at least.