The official site stats, "Games for Windows Marketplace will fully transition over to Xbox.com. Now you can get all of your gaming needs in one place. It’s convenient, it’s concentrated, and it’s a whole lot of great games."
To editorialize a little, it's about time. Xbox.com's Marketplace is a smoothly running, oiled machine, while Games for Windows Live Marketplace hasn't. Consolidating both will make things a lot easier to find content - and better yet, have a lot more PC games with Live Achievements on them.
Next up: a Live message tray that allows you to message and voice chat with PC and Xbox 360 gamers.