I thought it got changed to just one check each time you launched your game...which means you still can't play it if the servers are down.
Of course, that can still happen with Steam if offline mode doesn't work properly (I think I used to get the "this game is not available" message in offline mode for some games like Portal because I hadn't synced or something). It can probably even still happen in Steam if servers are just busy. It typically happens around releases (Portal 2), and I think someone was recently complaining about black ops.
Presumably Steam's servers are more reliable than most people's servers, but still, if they go down, you either can't play your game or it can become a crapshoot in a few cases (again, unless the offline mode works and everything is synced properly).
Also, in TF2's case, if a server loses connection to servers midgame, everyone can't use certain items (typically can't switch their items at all).