I keep hearing about how hyperinflation is about to hit. Surely in an efficient market, if this was truly likely, its effects would already be being felt? What is the rationale behind its prediction?

Goldbugs think any fiat currency will eventually hyperinflate. Note this is an unfalsifiable hypothesis.

Monetization of massive US debt, particularly due to the present value of the future liability of social programs. If current spending patterns don't change, monetization is the only alternative to defaulting on Treasury obligations, unless we begin cutting programs (ex. military spending) to help fund those liabilities or reduce those liabilities by reducing social program benefits.

Given what just happened in the RMBS and CMBS market, it's hard to claim that we live with an efficient market, particularly with respect to long-term trends.