Turn 10's Dan Greenawalt told us today that being required to connect to Xbox Live before you can play is "not the plan." You just need to connect online eventually in order to complete the career mode.
"In the beginning of the game, the player will be racing against Drivatars that are contained on the disc and were trained by the [Turn 10 development] team," Greenawalt said. "While playing, the player will be asked to connect to Live in order to download a completely fresh set of player-created Drivatar opponents – it’s not a lot of data, but the download contains enough Drivatars to cover all difficulty settings. Connecting will also download new content – but the vast majority of the launch content is contained on the disc.
"This download is simply additional cars and tracks. The gameplay is not blocked while downloading the additional content; it’s done in the background just like disc install."