Player A has Sneak Attack on board. He uses it to sneak in an Ulamog, the Infinite Gyre. His opponent casts Word of Seizing on the Ulamog before combat.

Since Sneak Attack stipulates that the creature is sac'd at the ‘beginning of the next endstep’ and word of seizing keeps the permanent until the ‘end of turn’ does that mean that the owner of the Ulamog will not have to sacrifice it ever?

Edited Scott Marshall (Jan. 24, 2013 01:47:59 PM)

Gatherer entry for Sneak Attack

10/4/2004 You only sacrifice the creature if you still control it at end of turn. If that creature has left the battlefield and come back, you don’t sacrifice it.

The delayed trigger is controlled by the player that activated Sneak Attack, it will trigger in the next end step (likely of this same turn), but he won't control the creature to be sacced so it won't be sacced by the trigger (you can't sac permanents controlled by another player).

When Word of Seizing's control effect ends, he'll get Ulamog back and Sneak Attack's trigger has already triggered (and resolved) so it won't trigger again for Ulamog.

Rules

603.7e If an activated or triggered ability creates a delayed triggered ability, the source of that delayed triggered ability is the same as the source of that other ability. The controller of that delayed triggered ability is the player who controlled that other ability as it resolved.

603.7b A delayed triggered ability will trigger only once—the next time its trigger event occurs—unless it has a stated duration, such as “this turn.”

701.14a To sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent he or she doesn’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action.

Edited Todd Bussey (Jan. 24, 2013 02:04:49 PM)

According to 701.14a
To sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent he or she doesn’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action.

The player who activates sneak attack controls the delayed triggered ability that causes the creature to be sacrificed. Since you can only sacrifice a creature you control, when the trigger resolves it will try to sacrifice it and do nothing since Ulamog is not under his control. When word of Seizing duration ends, the Ulamog will return to its owner's control. Since the delayed triggered abilities only trigger once (at the next event), the creature will never be sacrificed.

The rulings on gatherer confirm this "You only sacrifice the creature if you still control it at end of turn. If that creature has left the battlefield and come back, you don’t sacrifice it.

Here is the relevant text in regards to controlling delayed triggered abilities:
603.7e. If an activated or triggered ability creates a delayed triggered ability, the source of that delayed triggered ability is the same as the source of that other ability. The controller of that delayed triggered ability is the player who controlled that other ability as it resolved.

(Edited the original post to use card links…)
Note: this is the Official Answer.

From Gatherer: “You only sacrifice the creature if you still control it at end of turn.” That ruling predates the change from “at end of turn” to “at the beginning of the next end step” - but it's still accurate.

When Sneak Attack's controller begins their end step, that delayed trigger goes on the stack; on resolution, he no longer controls Ulamog, and can't sacrifice it. Then, during the Cleanup Step, Word of Seizing wears off and Ulamog returns to its original controller.

And, since Sneak Attack specifies “the NEXT end step” in its ability, it won't trigger again on subsequent turns.

Short version: yes, the Sneak Attack controller gets to keep Ulamog after that turn ends.

Thanks! – Scott Marshall, Magic Judge NetRep, L5, Denver
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Edited Scott Marshall (Jan. 24, 2013 02:13:12 PM)