(but different games. if I want to play TF2 and the family member wants to play Dragon Age, I don't see why that shouldn't be possible!)
Heck, you can already play single/multi player games simultaneously just by sharing passwords and using offline mode. Color me unexcited.
Here's even a quick config file change you can make so that your Steam always starts in offline mode and doesn't force-logout the other person already logged in. http://forums.steampowered.com/forums/showthread.php?t=25474... (tried this with my dad and it works great as long as he doesn't need to patch).