@Aaron:

Maybe I should have said their SERPs are their free content that attracts the vast majority of their traffic (that they monetize then) to the website.

Similar to your blog posts here on SEObook (or the guests posts or those of Peter) - they're what keep the traffic coming to your blog, I assume...and thus an analogy to google's SERPs on their www google com website.

if the quality of their SERPs goes down, they lose market share...if it goes up they win market share...(technically, I know its a bit more complicated in reality)...thus their SERPs are basically there to attract free traffic to the "website".

I think I need to think about the stuff that creates lock-in, that you just mentioned...I never really thought about that stuff toom uch to be honest!