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.
-- brion vibber (brion @ pobox.com)