I don't mean that to be flippant or sarcastic. It seems a good landmark with respect to government intervention in the lives of citizens. According to Wikipedia's entry on libertarianism,
Although libertarians share a skepticism of governmental authority, they diverge on the extent and character of their opposition. Certain schools of libertarian thought offer a range of views on how far the powers of government should be limited and others contend the state should not exist at all.A self-identified libertarian can very well support license plates. Although the libertarian values his personal liberty very highly, and requiring license plates is clearly a restriction of this liberty, he can benefit from this restriction insofar as the presence of license plates assists in the enforcement of automobile-related property rights. It is arguably a reasonable trade-off.
Opposition to license plates might be founded upon a philosophical insistence upon some very minimal level of government activity, or a belief that any government is inherently incompetent and thus should do as little as possible, or a deep mistrust of government power and the potential for its abuse. Or there may be other foundations, and any of these may be defensible in some way. I would just want to know if, for example, someone's opposition to more uniform registration of firearms is actually specific to that issue. For that one in particular, I've seen a lot of opposition that seems to be based on the trust/power issue, but I really wonder if people in this camp oppose other policies (like license plates) for the same reason. If there is inconsistency there, I think that it is worth exploring why someone is very mistrustful of government in one sphere but not another.
The license plate question could help identify what we're really talking about: is it a specific thing, or a manifestation of a more general thing?