Though developer DICE has decided against releasing mod tools for Battlefield 3, and also apparently encrypted the key files like mad, a modder by the name of Kiwidog has managed to churn out a night mod for the game. It's not extremely sophisticated, but given what he's had to work with, it's impressive and looks plenty good enough to be worth trying, if only to mix things up a little.
The bad news is this appears to be a "friends only" thing, at least for now, so unless you know the guy or can somehow find out whatever server or servers he's playing on, you're out of luck. Naturally, I'll be sure to let you know if I hear anything more on this front.
Anyway, so much for DICE's repeated "the engine is too tricky to mod" line, because if it can be done without any mod tools, surely it can be done even easier with.