Acquired distinctiveness is a status that typically requires a period of five years of sole use that is uninterrupted by third party use of the trademark. As Apple's legal team is surely to use their vast resources to ensure their marks remain solely used by and associated with Apple, it is likely they never have a problem with third party usage. On the other hand, there is a point where you cannot offer such protection to any marks as it becomes so synonymous with a product or service it would be akin to allowing Ford to have exclusive rights to refer to a line of their cars as "stationwagons".
Therein lies the give and take is such determinations, and is the reason that while Apple is likely safe for the time being, there may come a point that putting a mere "i" in front of otherwise generic word will no longer allow Apple to monopolize such wording as PAD, POD, PHONE, etc. It is one of the paradoxes of trademark law and brand protection, that especially those marks that are most recognizable and strong are at the same time the most vulnerable to such challenges based on genericness (see Google and the recent challenge in Federal Court over the genericness of their search engine trademark, and see also TiVo's strict trademark guidelines for an example of intensified policing efforts). These companies use vast resources to ensure they do not suffer the same fate of brands that suffered a complete loss of highly valuable trademarks (see BANDAID and THERMOS for examples).
An article by the LA Times recently published discusses Apple's most recent challenge to one of their most valuable brands, one that is sure to be a highly valuable IP asset for years to come, and even more so as tablets lead the charge as one of the devices looking to replace traditional computers.
