Can a site admin know when a new user has activated their account?

It is - easy to dump to csv with something like this (but possibly less ugly!)

for member in context.portal_membership.listMembers():

row =  member.getProperty('id')

row = row + ', ' + member.getProperty('email') 

if not member.getProperty('login_time').strftime('%Y') == '2000': 
    row = row + ', ' + member.getProperty('login_time').strftime('%c') 
else:
    row = row + ', No Login'        

if not member.getProperty('last_login_time').strftime('%Y') == '2000': 
    row = row + ', ' + member.getProperty('last_login_time').strftime('%c') 
else:
    row = row + ', No Last Login'        

row = row + ', ' + member.getProperty('fullname')

print row

return printed