Justin, Claudia and Matt have offered solid arguments. I would like to suggest, however, that we are asking the wrong question. Inflation is not theft. But is inflation fraud?
In a free society with private currencies, contracts between currency providers and customers would establish whether inflation amounts to fraudulent behavior. Of course, private currencies are effectively banned in the US--the government is the sole provider of legal tender. So in order to claim that government sponsored inflation is fraud, we would first need to show that the government has an obligation to provide a stable currency. Does such an obligation exist? I will leave this for you to decide.