I don't know about reliable statistics, but here's my anecdote: the top three user agents requesting files from inkstain this month self-report as bots:

The bots seem to account for at least 30 percent of the file requests received by inkstain. I'm getting an average of 95 hits per day on my robots.txt file.

I'm not complaining here. Many of the bot requests are coming from RSS aggregators, which means there are human readers on the other end of the process. And I adore the way the search engines have made the web so much more usable. Just more of an observation.