I've only seen it to be a gateway drug in people looking to fuck themselves up from the start. If you have an addictive/self-destructive personality, you're more likely to seek out new ways to get high. I smoked weed every day for nearly 6 years, didn't touch another drug, even when everything (save heroin) was offered to me. It's a matter of personal conviction.

Oh, and the cure for wanting to do heroin (don't misread this as "the cure for heroin addiction") is the scene in "Trainspotting" when Ewen McGregor is coming down. That was some profoundly fucked up looking shit.