The only 'evidence' for two-handed wielding comes from the weapon section of the book. How then do you explain the Halfling conundrum I posted earlier?
A quarterstaff is a two-handed weapon. Halflings cannot wield two-handed weapons, yet they are not barred in any way from wielding a staff as an implement. This suggests that, even though a Staff can be used as a quarterstaff if required, and uses those rules when doing so, it does not act as a quarterstaff when wielded as a weapon, or the Halfling would be restricted when wielding a Staff as an implement.
First off the bit about staves being quarterstaves when used as a weapon is not from the weapon section, but from both the implement section (under equipment) and the magic stave section of the PHB.
There is no question that a halfling could not use a staff as a weapon because it's too big for him. As for him using it as an implement, well if it counts as a two handed wepaon while being used as an implement then no he couldn't use it as an implement. This doesn't change anything because I am unaware of any rules to either support or deny halflings being able to use staves. I suspect this is something that rarely came up in playtest given that halfling wizards are likely to be rare.