Microsoft announced yesterday that Games for Windows that are a part of Games for Windows Live (used to be called Vista Live) will work like they had the gold version of that subscription.
Originally, if you had Xbox Live Gold, you would have Games for Windows Live Gold as well, and you could use the same gamertag, and your gamerscore and achievements would be combined on your PC and your Xbox 360.
Now, you don’t have to pay for a Games for Windows Live Gold Subscription, you can play with all the features of Gold for free.
If you don’t have an Xbox, and play a Games for Windows Live enabled game, you can play with gamers on the Xbox version of the game, and can play in Matchmaking games that use TruSkill (which still has flaws).
This doesn’t mean that Xbox Live Gold Subscriptions are now free. This only applies to PC games.