Safari claims 200 million installed base, WebKit at 500 million

Safari continues to lead the pack in performance, innovation and standards support," said Philip Schiller, Apple's senior vice president of Worldwide Product Marketing. "Safari now runs on over 200 million devices worldwide and its open source WebKit engine runs on over 500 million devices

via AppleInsider | Apple releases Safari 5 with extensions, expanded HTML5 support.

Safari's installed base includes Mac, iPhone, iPod touch, iPad as well as the number of Windows versions.  Assuming 45 million Macs, there are probably around 50 million PCs using Safari.

WebKit includes all iOS devices as well all Android and Symbian/S60 and Palm Pre.