As per @phantom42's comment, the actual enchantment we see placed on Mjölnir in the Marvel Cinematic universe is:
Whosoever holds this hammer, if he be worthy, shall possess the power of Thor.
It's possible that Mjölnir has some non-enchantment-based property - e.g. mass, magnetism - that makes it impossible to lift it without "the power of Thor" (whatever that is).
However, I think the events of the movies make more sense if the hammer has no such property, but the enchantment also prevents the unworthy from wielding Mjölnir.
The word "wield" implies control and intention. When Thor drops Mjölnir while on board the Helicarrier, the pilot isn't using Mjölnir for a purpose just by controlling the vehicle with Mjölnir aboard, so I don't think he/she is wielding it. Hence the enchantment doesn't need to restrict the Helicarrier's movement at this point.
In contrast, when a truck driver was attempting to extract Mjölnir from the ground with his truck in Thor, the enchantment wasn't having any of that, as the driver was trying to control Mjölnir - i.e. attempting to wield it - via his truck.
The lift/wield distinction is discussed further in answers to other similar questions:
And the Marvel Movies Wikia describes the enchantment in terms of wielding, although it doesn't provide any sources.