(disclosure: I'm the SSL co-op guy)

All of Google's free services (CDN, DNS, Chrome, etc) are centred around improving the experience of using the web, so more people use it for longer (and, hopefully, do more searches with Google and/or visit more webpages with AdSense ads on them). I'm not sure how more SSL-secured traffic feeds into that, exactly, but if anyone at Google wants to talk sponsorship, I'm all ears...