Microsoft has sent out a email containing a free 48 Hour Trial code. It says "Stand Defiant, and get 48 hours Gold. I don't really need mine, but I figured it was worth posting so if someone does need it, they won't delete the email.
With the 48 hour gold cards, you don't get the full benefits of gold. In fact, you pretty much just get the ability to go online and play against other people. It is very limited to give you a taste of what you could buy. At least you get some online playtime in though.
It's only if you've already signed up for live with a legitimate email. I'm sure a few people like to use fake emails thinking they won't actually use the account. No registered account with a good email though, no code.