This plugin looks almost like what I was looking for, but I have 2 suggestions.
1. Allow more then one IP (probably comma separated) to allow more then one developer/tester in more then one location, globally to test the site (this feature I dont need, but seems quite useful).
2. Allow a selection of Role to allow limited login access on a live development install parallel to a production install. Currently any user who knows how to type in the login URL can still login.
I have plans to implement feature #2 to this plugin. Id be more then happy to share my results with you.