Considering that anonymous users coming from Google make up 90% of Stack Overflow's traffic, maybe there are ways to track how satisfied these users are?
Since upvotes aren't an option to unregistered users, my thought was a simple, non-intrusive "Was this helpful?" box below answers, that would be shown only to unregistered users who arrived at the page from a search engine.
These "helpful" votes could be translated into upvotes for the user who provided the answer. Maybe for every 10(?) helpful votes, the user gets one upvote. I'm not sure what you'd want to do with the "not helpful" votes other than keep them as statistics. (Show them to 1000+ rep users? Use them as indicators of potential spam? Use them as indicators of a question lacking a good answer?)
This would be good for simple questions, that get answered and forgotten quickly. I think the user who provided the correct answer that helps lots of unregistered users should get some rep for that. It is desired behavior that should be encouraged!
Another idea: if rep goes to the person who provided the helpful answer, maybe some rep could go to the person that asked the question as well? (Maybe one upvote on the question for every 20 helpful votes to answers to that question?) After all, if the question is not written well, no one will arrive at the page from a search engine. This means that well-worded, SEO friendly questions are encouraged.
Obviously, this means you'd have to account for people who are trying to cheat. I think it shouldn't be too difficult to spot anomalous vote patterns, given the experience the SO team already has with that type of thing. Another problem would be SEO spamming (i.e. throwing keywords into the post for no apparent reason other than to increase the chance of a Google hit), but I think the existing flagging mechanisms will control this well. Also, if the question isn't what the user was looking for, then they probably won't find it very helpful anyway.