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Various sites are offering these codes but this is your best way to get them. I have extras for those unfortunate not to get any. All major siites have be offering keys
I signed up for this through a site that James. linked me too a few days ago. Not sure where it was, though. I got an email the same as what Lachrymose posted. Does that mean I'm guarenteed a code?
If you signed up through the Blur website I would think that you are guaranteed one because the message on Step 3 and in the confirmation email it says
On March 8 we will email you your Xbox token to download the beta as well as a 48 hour free trial of Xbox Live Gold.