There used to be a review here for the Roam Mobility Global SIM Card - a product which is no longer offered as of Jan 11, 2012. Previous Roam Mobility World SIM customers were given the option of transferring to a company called GeoSIM on that date, and a new US-focused product was launched .
I haven't used the "current" Roam mobility service so I can't comment on its quality / utility.
My current solution for US roaming is a prepaid T-Mobile sim card I bought from a generic cell phone re-seller the US for about $6, and just recharge it on an as needed basis. I got a free VOIP number online (available from Google Voice, etc...) which I forward to the prepaid SIM, so if I forget to renew my prepaid USA card and it expires I can just buy a new one and still keep the same number.
This system works well for my needs - more info on the "new" roam mobility can be found here: http://www.roammobility.com/ , and T-Mobile's prepaid rates can be found at http://prepaid-phones.t-mobile.com/pay-as-you-go-plans