If a touch spell has a duration of "instantaneous", that means you cast the spell and then hold the charge until you hit something with a touch attack. Or until you grab something else with your main hand or cast another spell.
That was how I thought shocking grasp was supposed to work. But I have no idea what "permanent until dispelled" means. If they truly meant that you gained the permanent ability to deal touch attacks that do shocking damage until somebody dispelled that ability (which I'm sure they didn't mean--that would be absurd), then the duration should simply have been "permanent". "Until dispelled" is implicit. If they meant you hold the charge until you succeed at a touch attack, the duration should have been changed to "instantaneous".
If there's no good explanation for what "permanent until dispelled" is supposed to mean, I guess I'd just like to see a consensus that WotC loused this up in the errata and it should have been "instantaneous".