can you dimension door through areas of null magic?
Not that that has anything to do with the argument, just a sudden thought.
I'd say to initiaite combat it'd be a surprise round if the DDers knew where they were teleporting to, ie line of sight which would allow at least a chance for the people being attacked by DD to see the person doing it. Inside combat, the people would have to delary their actions which lowers initiative for the entire battle, which isn't too bad but could cause some problems.
I think I'd allow it as is, you just have to remember that a person's weight isn't just their equipment, you have to add in the weight of the person themselves. Currently in my campaign (9th level), the wizard can just barely transport two other party members, their general gear, and the wizard's gear with teleport or DD, so I don't see transporting a bunch of heavily armed fighters in with one go being TOO much of a problem. Then again, surprising people with a bunch of halfling rogue's seems to be an interesting way to spin this.
Also remember that unless there was a plan set out before hand or the wizard has some way of communicating with the party without the attackers hearing, DD might not be the most surprising tactic in combat.
But rambling aside, Surprise round as normal, and people can delay their actions until after the dimension door.