I've setup a special watch for user agents that only hit robots.txt. That's because I thought I was spotting a trend where a ua only gets robots.txt and then returns days, weeks or months later without bothering to read robots.txt again and usually disrespects robots.txt. Turns out there does appear to be a trend. When I've got enough data to present a convincing case I'll publish it, but I recommend you keep an eye on these types of visits. :)
