"Microsoft says the restriction is designed to ensure that users get the best possible performance from limited netbook hardware."

Because being able to run more than three applications at once makes your hardware less efficient?

(EDIT: To be fair, I suppose that having a low fixed number of applications could let you pull off some useful optimizations. However, most of those optimizations seem like they would significantly change the OS architecture, so I doubt that Microsoft performed them. Maybe they did, though.)