Appertaining to the vagueness of the reason for Nexus attacking Undertaker, it wasn't even planned prior to the event. 'Taker needed time off for surgery and losing 3 consecutive PPV matches would make him look pretty week. However, having had Nexus interfere could turn out to be a very good thing if they do decide to go with Undertaker vs. Barrett. But it all depends on how soon, if at all, 'Taker can get healthy and get back on TV. It would give them a starting point for a feud between the two. With an appropriate build, I think it could be a successful program.
If I have my preference, however, I would much rather see 'Taker vs. Sheamus. I'm a mark for Sheamus and I really believe he has only just begun to tap into his potential. If WWE does decide to unify the titles, then I think it would be important to have more than just a one-on-one for the new strap. Cena vs. Orton vs. Triple H vs. Barrett vs. Jericho vs. Edge vs. Miz (yes, really has gotten that good) vs. Punk? Elimination match, maybe?