Is extremely viable. I've played about 10 games of it and with this build I've carried about 6 of them, others I was still testing builds with it.
First, I start machete + 5, then rush Spirit pf the Ancient Golem. Next, I grab boots of Mobility to help roaming around and ganking, as Leona's base movement speed is rather low. After my jungle item and boots, I seek my core items, Frozen Mallet, The Black Cleaver, and Iceborn Gauntlets. Finally, you can choose to have a more offence oriented defensive item, Sunfire Cape, or if your team is on the squishy side, pick up a warmogs.

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Gambit did it once I believe.

Excellent thread from a guy that went to platinum doing 90%+ jungle Leona http://na.leagueoflegends.com/board/...t=leona+jungle

I was shocked at how much damage I could squeeze out of her as a long-time Leona support player. The double AA reset on her Q especially was news to me.

Jungle Leona feels a lot like Vi to me, but swapping damage potential for more CC.