The only thing I've seen so far is that you can use the cars of any of the nine rivals you've beaten in singleplayer in multiplayer races. The advantage in using them is the rival cars already have the three upgrades (health/fans/speed boost) compared to their normal counterpart.
 
Beyond that? Nothin'. I'm kind of disappointed singleplayer and multiplayer aren't more intertwined, but hey, at least that's more reason for me to play multiplayer and ignore singleplayer, I guess.

Yeah, im perplexed the multi and single player aren't tied together more. It makes me debate which one  want to devote my time to, and i feel little incentive to take on the single player.
 
I hope the servers are up for you soon, the multiplayer is where this game shines.