I hate to generalise, but in my personal experience people that are somewhat less computer literate do indeed visit the homepage to search. I've seen my parents do it countless times, even though I told them searching from the address bar is possible. My guess is that precisely these people are less likely to know about SOPA and its implications.
It raises the question, would these people notice the logo change and would they even be bothered to check out what it means?
Hmmm that's a good point. I already know what I'm looking at, and the significance, so I'm not really in the target audience. I also don't live in the US, so I had to proxy through one of my servers to see what the effect was like. That brief exposure I had probably isn't quite the same as seeing it throughout the day, as Americans will.
It raises the question, would these people notice the logo change and would they even be bothered to check out what it means?