Craigslist is famously aggressive about scraping, too. While helping with some university research, I found that their anti-scraping defenses were really difficult to get around.
They've also been litigious about this in the past, so this project would undoubtedly get a cease-and-desist if it ever became a blip on their radar.
Which was a shame, because padmapper stuck it on a map and linked to Craigslist, instead of just taking the content. If Google Maps did it nobody would think twice.
I was a contractor at a company where one of the divisions did Craiglist scraping as part of the business model and they had a closet full of laptops doing the scraping part of the job - thirdhand what I heard was separate PC's were necessary for the anti-scraping workaround they were using.
They've also been litigious about this in the past, so this project would undoubtedly get a cease-and-desist if it ever became a blip on their radar.