For me, the same thing it took for me to buy a Playstation 3, an exclusive new IP that's new and fresh and exciting, and right down my alley. That game was Demon's Souls.
Such a game does not yet exist yet for the PSV, nor is it announced (to my limited knowledge). Maybe XCOM : Enemy Unkown, because if that's coming out for PSV, that'd be freaking amazing!
What about you? What'd it take for you to pick up a PSV? A specific game? Or a game that has some kind of je-ne-sais-quoi - you'll know it when you'll see it? A pricecut? The inevitable redesign? A change to a more mobile lifestyle?