NOT all Straight Talk "g" labeled phones will use the PREPAID AT&T coverage. What phone are you interested in getting?
Go to your local Walmart store that sells the ST phone you are interested in and on its box look for the bar codes that would be scanned for activation. At the bottom right of that white label will be either the letter "A" (for AT&T coverage) or the letter "T" (for T-Mobile coverage)!
As far as roaming goes, when I went to Canada I lost signal and service within a mile of entering the country but my brother's phone (who's under contract with Verizon) still had signal the entire time we were there and was be able to make calls at $2/min!! Besides that I'd say ST's AT&T prepaid coverage includes most metro areas in the USA :-)