Well, except for holy symbols, implements actually need to be held in your hand - although you probably didn't intend as much in your definition of "touch", one can "touch" something while holding something in your hand.
That said, I myself would rule that "touching" an implement is not enough - one has to hold it. The difference (in my mind), is that to touch it you could have it hanging around your neck and simply reach to it as a free action. To hold it, one would need to "draw" it in the same way as a weapon. The crunch issue behind the difference in philosophies is basically how easy it is to switch implements/weapons - so, either a free or a minor action, plus whatever actions are required to put away what the character was previously holding.
Now, holding it is not necessarily pointing it - so, one may not need to hold a greatsword with two hands to use it as implement. That said, the only characters I can see spending the feat to get a two handed weapon as an implement are those who intend to use it as a weapon (so, with two hands) - really, there is no game benefit (and indeed a disadvantage, as you need to spend a feat) to taking such a weapon as an implement unless one also intends to use it as a weapon. As such, I see the one handed greatsword implement argument as pretty much a purely intellectual one.
Now, from my own view on the cinematics of things, when using a weapon as an implement, I imagine the "power" coming off of it either from the tip (so, pointing) or from the blade - both, from my own "rule of cool", requiring a two handed grip for a two handed weapon (with weapons that you can hold in the middle, like spears and double weapons, being a posible exception). That said, when the drama of the scene calls for it, I can imagine a hero, through great effort, pointing the weapons at a foe and chanelling their power through it - movies (from Sarah Connor with her Spaz shotgun in T2 to the "childhood friend" in the chinese martial arts film The Empress and her Warriors with a two handed sword and a longsword when he is really pissed) are repleat with examples of an injured/angry hero with a two handed weapon continuing on the fight wielding it in one hand.
So, my own gut check is that most two handed weapons need 2 hands to use as implements, but that in moments of high tension I would allow its use with one hand. That said, as the only characters I can see using such an implement would do so because they also intend to use it as a weapon, I really only see such happening if they need to draw a potion or a dagger or something.
