Best USA prepaid SIM for very occasional data use?

[inquisitor]

That one supports UMTS on 850, 900, 1900 and 2100 Mhz, of which only 850 and 1900 MHz are used in the US. Apart from few regional carriers only AT&T and T-Mobile run UMTS on these frequencies, though T-Mobile has only recently started deploying UMTS to their 1900MHz spectrum and so coverage is limited. For an indicative coverage map of T-Mobile's UMTS1900 network see Sightings of T-Mobile 3G/4G/HSPA+ coverage on 1900MHz (PCS/UMTS band II)