Is Apple's Old Slogan, "Think Different", grammatically incorrect?

Not too long ago, Apple Computer used the phrase "Think Different" as an ad slogan. Is this a grammatical error (that is, it should be "Think Differently"), or are they trying to say something else (and what would it be)?

Merriam-Webster claim that different as an adverb dates at least as far back as 1744.

Everyone's assuming that this is "think differently" with the -ly dropped off, but note that there are also formations like think fruit or think pink, and "think different" could belong to that group. In other words "What should I think about this product?" "Think (that it is) different".

Anyway, "think differently" is a terrible slogan.