Cheapo pre-paid USA SIM?

We’ve got a trip to the USA planned and coming up soon.  My somewhat nifty smartphone offers one feature most others don’t:  dual SIM cards (one for work, one for personal use).

I’d like pre-paid flat rate GSM service for about 10 days in June in a densely populated urban area in the middle of the USA.  It would be nice if that option included the possibility to retain unused credits and a phone number for use months later when visiting other parts of the country.

I’ll do my own homework too, of course, but I figure plenty of tech savvy expatriate continent-hoppers must have already found the optimum solution.

I seem to recall that AT&T and T-Mobile USA are the only big-name GSM operators in the USA.  Is that still true?

What are my options here?  What (and when) did you try that worked / didn’t work for you on a trip back to the USA with a German-issued smartphone?  (Other than disconnecting my life from the internet, of course…)