Hoping to counter a slowdown in subscriber growth rate, Comcast saidThursday that it will begin rolling out a superfast "wideband" Internetservice that wipossible to download a full-length movie in standard definition in aboutfive minutes. The DOCSIS 3.0 service, which Comcast calls "Wideband Extreme50," will be deployed in a handful of markets, including Boston,Philadelphia and New Jersey, during the next few weeks and in the top tenmarkets where Comcast operates by the end of the year. The service comeswith a hefty price tag, however -- $139.95 a month for homes, $190 forbusinesses. While the service represents a major improvement for cable, itis already exceeded by cable services in Japan, where speeds of 63 Mbps arecommon.