Yeah, we've gone round and round the mulberry bush on this one...
The worst part is, just because you got a CS ruling that says one thing, don't think other people haven't gotten CS rulings that conflict with that. Obviously CS can't figure it out any more than anyone on these boards can. So it is going to be a matter of personal DM preference, at least unless and until there is an errata of some kind.
My 2 cents worth is, if the sustain provides the caster with the ability to launch an attack (and it was a ranged or area power) then it provokes an OA. So the Bigby spells would all provoke OA I believe. On the other hand, sustaining a zone which causes damage, even if you have to roll to "hit" creatures in the zone to do the damage) will not provoke OA because in effect the caster is simply maintaining the zone, not directing an attack at whoever is in it. The distinction being it is the decision of the target to stay in the zone or move into the zone, and that is what is causing it to be attacked. The caster isn't specifically launching an attack at the target(s).
Granted, that isn't the simplest possible interpretation, but I think it most closely matches the RAI. I really don't think it was intended that you would provoke an OA simply because you sustained a power. Also my interpretation avoids some ugly problems like what if a power creates a zone which makes a to-hit roll on a creature in the zone during that creature's turn? Suddenly the original caster is subject to an OA due to another creature's action? Doesn't make the slightest sense to me.