it was added back in the 3.3/4.3 days (http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=2670) . It's used in a number of places in the core as well as a number of the core modules. If you take a look at Signin.ascx.vb/Signin.ascx in the admin/security folder it should get you started.

Cathal

theres not a setting to enable/disable the captcha at the control level. However, there is a host setting to determine if it's used for security. This check is made during login to see if a captcha needs to be generated and validated - though as far as I remember, setting the host value pushes it to each portal.. Depending on your use case you may be able to also use the value, but normally you'd make it a module specific setting. If you want to use the portal value, you can retrieve it like so:

     Dim setting As Object = GetSetting(PortalId, "Security_CaptchaLogin")
                Return CType(setting, Boolean)

Cathal