http://www.pineight.com/rant/captcha/
Commons is listed under "confusing examples":
Wikimedia Commons links to Special:Captcha which mentions (but does not obviously link to) Commons:Contact us which links to Commons:Help desk, which permits anonymous posting. (Some other Wikimedia sites, such as English language Wiktionary, use the same CAPTCHA. The English language Wikipedia does not use a CAPTCHA at all.)
Presumably they're referring to signing up for a new account. (I think captchas are also triggered when anon users introduce new external links.)
Can anyone confirm or deny that enwp doesn't have the same captcha?? I thought all WMF projects had the same one now?
cheers Brianna