It would be nice, I'm not completely sure why Microsoft hasn't put more of an effort toward this, maybe even as an add-on to those games as a different network plug-in. There would have to be some sort of subscription service for Live for non-XBox players for PC users to join Live games, that or they would have to add an option that you can find and join a regular game by using non-Live resources to connect via the Internet to regular games on normal servers.
I was disapointed to find that this sort of ability wasn't included in the current Live package, I can understand the closed circuit for Live servers (keeps the riff-raff out), but it would have been good if they included the option to allow XBox players to join non Live games.