Artem Russakovskii, of the blog Android Police, wrote that he was using his new HTC Evo, when he noticed that the phone's normal 3G signal icon had been replaced by a 4G signal icon.
Because 4G coverage -- which boasts significantly faster speeds than 3G -- has not officially been launched by Sprint in the Bay Area yet, Russakovskii wrote that he was surprised.
Kristin Wallace, a Sprint spokeswoman, said that Sprint has not secretly unrolled the service, and surmised that Russakovskii had probably stumbled into a 4G pocket.
According to Wallace, last year Clearwire Corporation -- the company building Sprint's 4G network -- gave Silicon Valley smart phone application developers access to the network.
Wallace said those with the Evo and Overdrive smart phones, which run on Google's Android operating system, as well as those with Sprint broadband cards for their laptops, may periodically find pockets of 4G service throughout Silicon Valley.