For example, suppose I write a little tool using LWP::UserAgent or WWW::Mechanize (rather than LWP::RobotUA or WWW::Mechanize::Polite ?, say) that simply collects a number of web pages for me while I sleep. Is it illegal or unethical for such a scraper to ignore robots.txt?

Whether it is legal or not I won't get into, but AFAIC, I don't have any ethical objection to such a tool as long as it doesn't impose a greater load on the target server(s) than you would if you were to perform the same task manually.