Microsoft has issued a statement on this ( via @winrumor ) :
We are investigating these reports and will provide an update when we have more to share
Well there isn't really and they told him that it's because of some update to the marketplace and the update will take some time. a clear reason for this outage, but a blogger just had a chat with Zune and Windows Phone customer service
We do not clearly know the reason behind this, but we will keep you updated on this. Microsoft's silence on this is clearly not helping.
Update : ....and it's up.