It's not clear yet how single player Starcraft 2 will work, but it may be very similar. A Battle.net login screen will be the first thing you see for either mode. How it handles disconnects can't currently be tested in beta, as it's multiplayer only and the single player button is one of many UI elements grayed out.
Seems every major gaming company is trying to force online as a method of DRM, or a way to try and hinder used game sales. Recent EA games are intentionally leaving out pieces of the game on the retail disc, and require the user to go online to download the rest. They still call it DLC, but when it's free, locked to a code in the box, and available when the product ships, it's a bit fishy.
Whats really frustrating about all this is how it makes it impossible to play these games in certain environments, like an airplane (without forking over for internet access), or worse yet, soldiers out in the field seeking some RnR time.