Wolfster, I think you are on the right track. Since the BB did not change the action by calling then it means that UTG's action must stand. Since his action must stand then he has put in 50% of a raise and therefore MUST make it a full raise or $400.
Nick's last answer would seem to be wrong, the OOT bet DOES stay in the pot when the BB only calls the 200. And there is no oversized chip in this scenario. (we are assuming 3 x 100 chips, yes?) which makes it a 50% raise and binding as a full raise. (Rule 40, removing one chip does not leave LESS than the minimum bet so it can't be a call and it is 50% of a full bet - rule 38)
The intent of the OOT bettor does not change how the error is handled except after it is fixed and the hand finishes, then a previously warned intentional OOT bettor should be penalized.
Now, if the BB changed the action (by raising) THEN the UTG's bet comes back (rule 36: :...if action changes, the out of turn bet is not binding) and he has all options open to him. (again, a repeat offender is handled the same way as far as the bet and the hand, he just should be penalized after)