Eminem busts a contender for spewing written lines instead of rapping on the fly. One of four rap battles you can only see on the DVD.   B-Rabbit, a wannabe Detroit rapper, has a pregnant girlfriend and a dead-end job. He moves into his alcoholic mom’s trailer to save money and he chokes at a rap contest. Things improve but slide downhill again, including being mugged by rivals. Everything hinges on another rap showdown. Can B-Rabbit pull truth out of his cap? A rap version of "Saturday Night Fever." From trailer park to rap superstardom, Slim Shady's biopic looks like a gritty, white-trash wonderland with Wonder Boys' director Curtis Hanson at the helm.