Joystiq has more clarity on Blizzard's rationale for dropping LAN support in StarCraft II, which has inspired much outcry, as well as a petition from unhappy fans. The explanation is as many expected, which is they are using Battle.net to combat piracy of the upcoming RTS sequel. Here is the statement they received from Bob Colayco at Blizzard PR:

We don't currently plan to support LAN play with StarCraft II, as we are building Battle.net to be the ideal destination for multiplayer gaming with StarCraft II and future Blizzard Entertainment games. While this was a difficult decision for us, we felt that moving away from LAN play and directing players to our upgraded Battle.net service was the best option to ensure a quality multiplayer experience with StarCraft II and safeguard against piracy.

Several Battle.net features like advanced communication options, achievements, stat-tracking, and more, require players to be connected to the service, so we're encouraging everyone to use Battle.net as much as possible to get the most out of StarCraft II. We're looking forward to sharing more details about Battle.net and online functionality for StarCraft II in the near future.