As I mentioned in wikitech-l, I've started putting a captcha into production
testing on some of our sites. (At the moment: Meta, Commons, most of the smaller
Wikipedias, and a couple others.)
This is meant to preempt automated spam and, potentially, other automated
vandalism attacks. At the moment only edits which add new URLs to a page will be
stopped by the captcha; potentially other actions may be as well from time to time.
Obviously a captcha form will be a bit annoying to a bot. ;) Any registered bot
account is excepted from the edit captcha, as are sysops and accounts that pass
the 'semi-protection' threshold.
If you're operating a bot on the Wikimedia cluster which isn't logged in, or
uses a brand-new account which hasn't been registered as a bot on that wiki, be
aware that you might get hit by the captcha if your edits add URL links.