I think we can rule out Belle and Rumple’s kiss as the key to breaking the curse; it’s been made pretty clear that Emma is the key. The creators have pretty much confirmed it:

 During a confab with the series’ creators, we asked why, when Mary Margaret and David finally shared a smooch some weeks ago, it wasn’t a grand moment of enlightenment a la when Sideways Sawyer and Juliet touched hands. Edward Kitsis answered, “It wasn’t an ‘a-ha’ moment for them because the curse hasn’t been broken yet — and clearly they’re not the keys to breaking it. They were just the keys to making the savior.” So, maybe that savior – aka Emma – is the one who needs to share “true love’s first kiss” to start unraveling the queen’s curse? “That is an interesting question,” Kitsis allowed. “Unfortunately, Kathryn’s disappearance is going to take up a lot of Emma’s time,” meaning no lives-changing romance for the sexy sheriff just yet.

I mean, I love R/B as much as anyone, but making them the key after all the fuss made about saving Emma, and how she’s the savior who’ll bring back the happy endings… that would be pretty cheap, right? And it would mean Rumples lied to Snow in the Pilot, which I don’t think he’d do.