While I haven't heard about the "committed couple" rumor, it should be easy to eliminate a couple of people...
Jennifer and Cowboy...let's hope these two aren't in a committed relationship; also didn't we see Cowboy's fiance in the first episode?
Will & any female...unless Will is lying to everyone.
Karen...we can safely say that Larry is not in the house.
If anyone has previous episodes on tape, a good indication of who a "committed couple" can be is to do two things...
(1) Watch the first episode where the contestants are chosen and are leaving their families/friends behind.
(2) When someone becomes HOH, look at the photos in the room.
If anyone is shown with a boy/girlfriend or husband/wife whenever they leave, odds of that person being part of the "committed couple" are slim, unless Shapiro and Co. and CBS are going to great lengths to fool us (doubtful). Anyone who leaves without showing a significant other, is a possibility.
If any of the past HOHs (Jase, Marvin, Drew, Diane) do not pictures of girl/boyfriends or husband/wife included, then they too could be suspect. Wasn't one of Drew's pictures, a shot of his house? When I saw that, I thought, who brings along a picture of their house to a reality show like "BB"?
Also, several houseguests haven't really talked about significant others. Has Jase, Will, Scott, or Marvin mentioned a girl/boyfriend at home?
Although it could be a pipedream, but as suggested earlier, the "committed couple" could be Jase and Will. If I were going into a reality show with my girl/boyfriend with me, the easiest way to keep people from finding out is to maintain distance from one another, even act like enemies. Also, hanging out with two different groups of people will help as well. Perhaps Jase's flirtation with Holly was a mask, because he obviously has some "metrosexual" tendencies. Most heterosexual people don't ask other men to bathe or shower with them (as Jase did to Cowboy). Also, the "homophobic" comments made by Jase (at least the ones I'm aware of) have been pretty tame and lighthearted.
Lastly, I'm not sure why the producers would have a "committed couple" in the house. I understand the reasoning behind the twin switch - Adria gets a reward for fooling her houseguests: her sister gets to join the game. But what is the reward for fooling the houseguests about being a couple? If it is revealed whom the couple is, the other houseguests will likely seek to get rid of one or both. Notice how quickly the 4 Horsemen turned on Jase after Holly entered the picture.
Producers could start a second twist once the first twist is revealed. Maybe when Natalie enters the house on Thursday, she'll bring an envelope with her that reads: "Houseguests, there is a couple in a committed relationship sharing the house with you. It is your job to seek these people out and evict them before they have the opportunity to wind up in the Final 2."