Well this is a nice little feature.
According to MTV Mutiplayer blog, Blur gamers will be able to challenge each other across platforms and even from Xbox LIVE silver accounts. You can challenge friends on the game via Twitter regardless of whether they have the PS3 or Xbox 360 version.
The game’s lead designer explains:
In the single player mode when you finish a race, it records all of your stats and information. You can choose what you want to put into your challenge and send it to your buddy. He receives that as an Xbox Dashboard message, so he could be playing another game, he could be watching TV on Netflix.