Justin and Claudia both offered fantastic viewpoints on the question of whether inflation amounted to theft.
While I don't see it written as such in the article that Justin linked to, I believe the idea that taxation is theft comes from the following logical train of thought: Inflation is a tax, tax is theft, ergo, inflation is theft. Justin's point that we don't have property rights over the pecuniary value of an object is spot-on; going with that mindset, those in favor of categorizing inflation as theft also have to consider some other possibilities. The idea is that one's buying power is less; would changes in aggregate demand be theft as well? Let's say that cargo pants become a massive fad, and their price correspondingly rises. Since your buying power in terms of cargo pants has decreased, is this theft? By the previous logic, I'm not sure how it isn't. Further, when I think of theft, I think of a clear victim and a clear antagonist. Who's responsible for the cargo pants theft?
I'm also reminded of an argument that comes up in policy circles-- it comes up most often when a big box retailers move into towns. Existing businesses complain that these stores will take "their customers," as if they have a right to future business. No such right exists, and no right exists to the pecuniary value of an asset either.