Are Justin Timberlake and Jessica Biel on the rocks after three-plus years of coupledom? "Justin's New Betrayal," blasts the cover of Us Weekly, which quotes sources as saying the crooner is "smitten" and "absolutely obsessed" with Mila Kunis, his co-star in the currently filming (and now awkwardly titled) "Friends With Benefits." "Justin has strayed before and he'll do it again," maintains an insider, "but he has no intention of ending things with Jess." His rep, however, calls foul on the report, scoffing that there's "absolutely no truth" to it. Kunis, meanwhile, is supposedly off-again with Macaulay Culkin, her boyfriend of eight years, but an insider insists that the pretty, pocketsize starlet has no interest in Timberlake, for a number of reasons. "Mila would never go for a guy with a girlfriend," pooh-poohs the confidant. "And he's not her type!" Meaning, we suppose, that the chart-topper never hung out with Michael Jackson and Bubbles or outwitted Joe Pesci when left home alone as a kid.