capturing the validated username

Hi
I am using the sqlmembership provider with the login control from the toolbox. I want to capture the validated username after the user is authenticated.  I need the username so that when the user adds or modifies a record i can have an audit trail.  where is the best place to capture the validated user right after authentication? Ideally, i would like to capture it on the login form but how?
 
thanks in advance
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