The US$ isn't really a Ponzi scheme in itself since it will keep it's value if it's bought and sold in equal amounts. With the Yanks printing money like there's no tomorrow it approximates nicely, but now with worldwide attention to controlling deficits teh analogy becomes weaker.
What really IS a genuine bona-fide Ponzi scheme is every single Social Security system in any country worldwide. The 'return on investment' of the 'investors' who are cashing out now (old-timers getting their pensions) depend 100% on the new 'investments' of new 'investors' (Social security deductions from current workers). Since people cashing out take more than they ever put in, by definition it can only work if the pool of 'investors' (tax-paying workers) increases indefinitely. Textbook Ponzi.