If it's in arena, they will have a good amount of mp5, but no where near as much as they do in high end raids (unbuffed of course). The only talents that directly relate to mana regeneration, is a talent which allows 30% of mana regeneration to continue while casting (almost every resto druid should have this) and one that increases Spirit by 15% (generally not taken in pvp specs).
Both of those talents are faily weak in Arena, as all the arena gear, aside from gems (which are better used on other stats), has no spirit. This means that those two talents do miniscule amounts of mp5 regeneration in arena (when specced/geared appropriately).
While it makes it harder for mp5 regeneration to be high, they still get it through strait up mp5 gear (which most gladiator pieces have) and gems. These tend to pile up, giving the druid a good amount of regeneration, which can make things harder for the opposing team, but it makes innervate less useful.
This is easy to counter, but often not taken care of. In welfare epics, the average druid will regenerate enough mana to always keep a stack of lifeblooms rolling on one target, with a little left over. To take care of this, keep draining mana. It's obvious that one heal can make a massive difference on killing a druid, but if their mana constantly stays near 0, they will not be able to heal, or shapeshift. Since the resto druid (while in caster form) only has one spell that's mana-free, Nature's Grasp, they'll only be able to run around like a deheaded chicken (or pillar humper).