I've only heard of one prepaid SIM offering unlimited iPhone data in the US. I have never used it, but I did order a Digicel Jamaica from the seller and service was good.
Mr. Simcard - Now You're Talking. iPhone Prepaid Sim Cards USA, Worldwide.
VladS
Mobile phones: iPhone 5, Blackberry 9900, Nexus S, Samsung S3322 duos
Mobile data cards: Huawei E587u-5, Huawei E583c, Huawei E160
Postpaid SIMs: CA: Fido, Wind; INTL: Telna
Prepaid SIMs: DE: Fonic, Lidl; AT: yesss!, bob; UK: O2; US: AT&T; RO: Orange, Vodafone; FR: b&you, Lycamobile; NL: Lycamobile; BE: Lycamobile, Jim Mobile; CL: Entel; MX: Telcel; INTL: eKit Blue, eKit Yellow
Dead SIMs: too many to list