It's a long established fact that the people who have Xbox Live Gold are likely to be the people Microsoft reward. It's not a bad thing – although Silver members might disagree with me – and, considering the revenue brought in from Gold members, it's definitely not an unexpected thing. So the news that the Mass Effect 3 demo will require you to have Xbox Live Gold isn't a surprising thing.
However, Microsoft have decided that anybody who downloads the demo deserves to be able to play a great game like Mass Effect 3 and so will give everybody access to Gold for a limited amount of time (and if you decide to keep the service afterwards, well that's just rosy…). No word yet on how exactly this will be administered, whether it'll be something pre-loaded into the demo or if it'll take the form of a free Gold weekend, but this will likely be announced in the near future.
Mass Effect 3 will be available for purchase in early March; the demo will be available on Valentine's Day.