Good idea - here are the changes I have made.
NOTE: I am in a AD only environment - no one logs in with username/password. Some of this code may have changed since its posting, so let me know before code compile for updated samples.
Eliminate Domain Name from UserName - no need to have the domain prefix
Preserve Login Parameters - with the redirect through windowssignin.aspx, querystring information is lost - this is a fix for that.
Password Being Used in AD Environment - the user's password is not stored in AD in my environment but the password is used as part of the login process. Changed to only use the GetUser(username)
Modified AddLog stored proc to update lastaccess and lastactivity values since these seemed to have stopped working through code several revisions ago.
Related Stuff:
Switch User Module - if your network account is set as Administrator (or even edit rights on a page), this module will take care of switching between accounts to let you see the page as a normal user.
User Manager Module - I have an AJAX powered user manager which has allowed me to bulk add/remove users using AD and manage what groups they are in.