Funny enough, Steam "offline mode" requires an internet connection. When in "offline mode" Steam still sends and receives data necessary for its operation. Also, you cannot start steam without an active internet connection and you have to connect first to enter "offline mode" (unless they changed that behavior in the last 18 months). I've discussed this issue with Valve, and its the nature of steam where "offline mode" makes you appear offline but still authorizes your game licenses every 3 days if possible but it does it at minimum every 7 days. If it loses an internet connection it will enter "offline mode" itself but after 7 days it cannot verify your game licenses and will prevent you from playing. Valve's resolution was "wait until you have an active internet connection to play the game". I was without internet for a total of 11 days at the beginning of Janurary 2010, and for 18 days at the end of January to the middle of Feb 2010. Comcast replaced the line drop, splitter, and found a "minor noise issue" on the next power pole and ran a new line between there and my pole and stuff has been fine since.
Even if the repair cost me $190
In my case, I was able to play left4dead 1/2 for two days in single player mode without an internet connection, but after day 3, a pop-up came up when I wanted to play and told me Steam cant validate my game licenses and to make sure my internet connection was operational. I used to keep my computer running 24/7 and would only start steam when I needed it (eg to play l4d 1/2 & gmod) or just wanted to make sure the games were up-to-date.
Of course, its been a loooooooong, loooong time (in internet terms) since I've even used Steam so it may have changed. I don't know. The last time I ran Steam was to get "The Passing" DLC for L4d2 and to subsequently play it alone, in "offline mode".