I like it, although I'm not convinced that "think different" is proper grammar ("different
ly"), just as I'm not convinced "fun
nest iPod yet" is proper. But who cares.
Not to get off track, but the word "grammar" has at least two senses. These are
- prescriptive grammar: a set of more or less arbitrary rules that self-proclaimed experts dictate people should or shouldn't do with regards to spoken and/or written English (e.g., "don't strand prepositions at the end of sentences" or "don't 'split' infinitives by inserting elements between to and the bare verb," etc.).
- descriptive grammar: the actual rules within the language that govern things people can (or can't) say that do (or don't) make sense. A descriptive analysis can formulate rules (i.e., grammar) for why constructions like "the cat" are grammatical but "cat the" is not.
Most people blur the distinction between these, and linguists really only care about the latter, since they are the rules that actually govern the use of the language (although the former often has sociolinguistic implications).
In any case, I don't even think this is an instance of prescriptive grammar in action; I think your analysis of this construction is wrong.
Different is not a modifier on "think" (i.e., not an adverb); it is an adjective, a predicate, the theme of the verb, what
is being thought. (Compare adverbial "think quickly" with adjectival "think new".)
While either adjectival or adverbial interpretation is plausible, I think it's the adjectival interpretation that is intended. In any case, Apple's slogan is certainly not "wrong" by any standards--descriptive or prescriptive. (Although even if there were a prescriptive "problem" with it, a flat adverb, I'm not convinced it matters--and since there are many varieties of English that allow "flat" adverbs, i.e., with out the
-ly, there would still be no descriptive problem [not that there ever can be for sentences native speakers intentionally produce

].)