Do creatures come into play tapped if they come into play at the same time as a card which says "Creatures come into play tapped"?
No, your opponent's creatures will not enter tapped.

The ability "Creatures your opponents control enter the battlefield tapped" creates a replacement effect on creatures entering the battlefield. Rule 614.4 says


  Replacement effects must exist before the appropriate event occurs -- they can't "go back in time" and change something that's already happened.


Since your Urabrask wasn't on the battlefield before your opponent's creature entered, it's ability does not effect that creatures.