How does Civilization 5 interact with other Steam games?

Civilization 5 utilizes SteamWorks and Steam only allows you to have one thing running at a time.

If I buy a physical copy of Civilization 5, will playing it block me from simultaneously playing a game that I downloaded from Steam on another computer?

Does it matter whether I am playing online?

What if I purchase the game from Steam, will playing it in offline mode allow me to play other Steam games at the same time?

Playing Civ V in offline mode should allow you to play other steam games simultaneously. I feel compelled to point out, however, that steam only allows one copy of itself to run at any given time, so short of some technical tinkering, you're still limited to one game / steam / computer.

I don't think that there is a physical copy of civ V. Everything I've read indicates that its going to be exclusively digitally distributed.

As far as I know, Steamworks is going to function as the game's DRM which means that even if there's a physical install disc, you'll need steam to play it, even in offline mode.