GameSpot reports that Sega GT Online is in development for Xbox and due out in "the fall" (sigh). As well as allowing up to six players to race against one another, the game will encourage the customisation and tuning of each car, and offer 160 of them in total - from three different eras and including several prototypes. You can even trade or wager the pink slips for each before the start of a race, which could be interesting if you genuinely risk losing the car from your copy of the game via some sort of save data mechanism.
Sega of America veepee of marketing Mike Fischer seems to confirm this. "Sega GT Online takes racing for pink slips to a whole new level," he comments. "If you think you've got what it takes to be the best, you can put your hard-earned car on the line and square off with thousands of gamers online via Xbox Live. Hey, if you lose big you can always talk trash--or grovel and beg for your car back."