The tweet is more of a tease than an announcement, in spite of what they might say. "We've hit 50,000 followers, which means it's time for an announcement! Coming Fall: a new way to earn Rewards based on your Achievements!"
The Xbox Live Rewards program lets players earn prizes, including Microsoft points, for doing things such as searching on Bing, filling out surveys, and performing certain tasks on Xbox Live. And now it looks like you'll be rewarded for actually playing games. Maybe.
This isn't the first time something like this has happened. We'll have to wait and see if it'll be a sponsored, limited-time-only offer, or a long-term part of the program. One thing's for certain: It sounds more interesting and appealing than searching for stuff on Bing.