It means that that either: a) You mis-typed the username, or b) the player hasn't got a fully-paid-up minecraft account, or c) the mojang server that checks the user is too busy or offline, and unable to handle the request.
In the case of (c), you get an error message too, just refresh till it starts working again...
Couldn't be bothered to wait till tomorrow - now it'll put a 'warning' above the username entry box if Mojang's server doesn't respond, but it will still try to show the user details. ;)
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