Facebook will require all in-app transactions to use Facebook Credits
That doesn't mean that the in-game currency for every game is now Facebook Credits, though. Games can still count up gold doubloons, red gems, or anything else they want -- it's just that any exchange of real money for in-game stuff has to use Facebook Credits as the middleman. This is good for customers, because they'll only have to give their credit card info to Facebook, rather than sharing it with new, unfamiliar companies whenever they want to take a chance on a new game.
For developers, it's mostly a good thing, too. Yes, it takes time and money to implement Facebook Credits, but bigtime developers like Zynga (FarmVille, Mafia Wars) and Playfish (Pet Society, Restaurant City) have already switched over, and everyone else has 5 more months to get ready. Once everyone is on the same system, the developers will very likely profit from the new, lower barrier to entry. ("Well, my card's already in the system, and this game seems fun. Might as well drop 5 bucks on some new stuff for my character.")
Ultimately, Facebook Credits could expand outside of Facebook to become the new PayPal. Facebook's already a large company that hundreds of millions of people (sort of) trust with their sensitive data, and an upstart online business without a reputation would benefit from processing through a known commodity like Facebook.