I can't speak for the Pitboss functionality in Civ 5, but unless there is DRM similar to Ubisoft's craziness, any Steam game can be played offline.
Doesn't it have to make that initial connection first? I have not tried, but I'm pretty sure I can't fire up Civ 4 via Steam if I don't at least connect to their service once.
Motherboard: ASUS P5QL Pro LGA 775
Processor: Q6600 @2.4
RAM: 4 gig OCZ Platinum DDR2 1066
Harddrive: 250 gig Seagate SATA 3.0g
Video Card: ASUS EAH 4850 512meg
Operating System: Windows 7