This functionality is now build into latest versions of Umbraco
Updated to support Umbraco 7 - For Umbraco 7 use version 2.0
This is an extension to the default UsersMembershipProvider which adds lockout functionality.
This will help to reduce the chance of a brute force attack on the backend login area suceeding. I've blogged abouit this if you are interested.
This will lock a user out of the backend if they enter the incorrect password a specified number of times within a specified window. The user will be able to login when then window has expired.
You can set these values on the UsersMembershipProvider in the web.config. By default they're set to 5 attempts within 10 minutes.