Is it time for a web crawling code of conduct?
readwriteweb.com
readwriteweb.com
Furthermore, unless a crawler is trying really hard to disguise itself, site owners have the ultimate 'self-help' available to them: just block bad crawlers entirely.
Any formal code-of-conduct beyond what are already customary practices (like following robots.txt rules) thus strikes me as superfluous.
It also helps in distancing legitimate companies from illicit ones.