You dont show as 'online', but it does state on the website friendslist that you are browsing xbox.com. Not on the console though. Also, if you play an xbox live enabled game on a Windows 7 Phone, you dont show as online via the console or the website.
When I sign in on here if any of my friends are on the console it tells them I have signed in but on the dash my avatar is still asleep , it has confused one or two in the past when they invite me to games and I tell them I can't
'cos they think I'm on the console but signed in as ' show offline . '