Are there internet users connecting with HTTP/1.0? Such as over a proxy. Because I'm going to block HTTP/1.0 since most of the bots are using HTTP/1.0.
While it is mostly in use by bots and other crawlers, why not block bots the regular way, by adding/changing the robots.txt file (for the "law-obeying" bots) or by changing your .htaccess file to block the criminals. This site is one of many describing these procedures.
Update: If I were to write a malicious bot today, I'd use http/1.1, just to throw off people blocking 1.0 :).
Second, there may be some legacy browsers that may still use 1.0 (AFAIK all recent browsers use 1.1). These may also include accessibility-type browsers (like Lynx, or blind-support browsers) or browsers on phones/devices that cannot be updated (i.e. my Nokia 6310's browser uses 1.0 - luckily I've switched to a Blackberry). Blocking 1.0 might lose you that type of traffic.
But here's the real easy solution: monitor your logs for a while. If you don't see a lot of 1.0 requests - block it, safely assuming you won't lose too much traffic.