Games For Windows Live, the new service that integrates Xbox Live functionality with Windows PC games, is set to launch May 8th, alongside a port of the classic Xbox title halo 2. Major Nelson lays out the details:

Silver Membership Features
Single gamertag
Common gamer profile
Common gamerscore
Single player achievements
Private chat via text and voice
Common friends list and online presence
PC only multiplayer including browsing a list of active PC games

Gold Membership Features
All Silver membership features PLUS
Multiplayer matchmaking with friends
TrueSkill matchmaking
Multiplayer achievements
Cross-platform gameplay

It's an interesting idea but I doubt it will gain much traction beyond people who already pay for Xbox Live (who will get to use GFW Live for free). How important is being able to play against Xbox users to PC gamers?