Well technically it is a Trial subscription, but it allows you to deploy games to your Xbox 360. This is not a premium subscription, so you won't be able to submit games to the current beta site (for XNA Game Studio 3.0 CTP) or review other's games and you won't be able to download premium content like the Ship starter kit. But, being able to deploy a game to your Xbox 360 for free is definitely a good thing!
How do you get this free subscription? Easy - just sign up to be a part of the Dream Build Play competition. The competition is only for Xbox 360 games (this is why Microsoft is giving away the subscription trials). The grand prize winner will get $40,000! That is a nice chunk of change. The winner may also get to sign a deal with Xbox LIVE Arcade much like some winners from last year's competition.
So don't just stand there ... get the free subscription while it is available and start working on your entry!