Prezi Conference
Looks like there's another conference dedicated to presentation on the scene this summer in London. This one has a decidedly Prezi focus as it's being put on by The Prezenter, one of Prezi's largest training partners, so if that's your thing, then this conference may be as well.
UPDATE: Oops, it's seems like while there's no Prezi logo immediately visible, it is being completely organized by Prezi itself.