login information is handled by Microsoft Membership Component.

DotNetNuke modules all rely on framework login, if you want to replace framework login, you need to write your own authentication or membership provider depending on your needs.

There is a dnn object you can use to get this information.  The object is UserInfo.  In your case you want toe user name so you would want to call

UserInfo.Username

this should give you the user that is logged into your portal

hope this helps

Mike