A Risk-Taker's Debut.

LOS ANGELES NECK FACE banged a metal pipe on the side of a blacked-out doorway, jumping out at unsuspecting passers-by to shout, ''Aaarrgh!'' Banksy floated anonymously (or so went the rumor) around the perimeter of a room dominated by a huge cathedral window the graffiti artist had scrawled with spray paint. Skateboarders skidded off geometric ramps designed by Lance Mountain and Geoff McFetridge just inside the entrance of the Geffen Contemporary at MOCA as an art mob filed in from a brisk evening and up a ramp into the enveloping graffiti world of ''Art in the Streets,'' the first major American museum exhibition devoted to street art, and a first for an occasionally controversial museum director making a debut. [ABSTRACT FROM PUBLISHER]