I would argue that good aiki is in the cage, aikido people just can't see it anymore because it looks [so] different than what we practice. Look at the attack angles of George St. Pierre, or the center-line shifts Anderson Silva takes before he strikes. Look at how fighters break balance before executing a technique... They may not call it aiki, but many of MMA fighters can express their aiki better than we can - we just refuse to acknowledge some brute is more in tune with the namesake of our art than we are... Not to mention the fact that they can do it under a circumstance in which we can hardly perform..
I guess one definition of aiki - in this context, certainly - is yielding, and taking the path of least resistance by allowing the person to do what they want, harmonising with this intent, and adding your power to it.
Whereas what I see in the UFC is a lot of fighters with a wrestling base, who use brute force - in direct conflict with the strength of their opponent - hence the success of Matt Hughes for a long time, for instance, who would pick people up - while they actively used their strength to resist - and slam them.
Chuck Liddell is likewise not concerned with aiki as I understand it - he just gathers as much force into a fist to someone's face, regardless of how 'strong' their face is...
I've seen plenty of things in the UFC where i think: 'He should allow him to push his wrist, as he has put such a lot of strength into doing so, that if he removed his resistance quickly, that would unbalance him' etc. - but instead they struggle against one another, not achieving the harmony which, in my opinion, and a lot of others', is what aiki(do) is.
I was interested with the matter of using atemi to create openings/destroy the posture of somebody before applying a technique when I saw Cro Cop do exactly that to get the choke on that guy on Saturday.
It worked then (beautifully), but i've seen plenty of instances in the UFC where people attempt the same choke on an opponent who can still actively resist. Aiki (I think) is not about doing what you want - it's about doing what your opponent/partner wants: you 'yield' to them, as Gozo Shioda said - you have no intent: you assist the other in their intent, but in the process bring about a different outcome.
When GSP and Anderson Silva can do it in a(n actual) war, like O'sensei, Koichi Tohei, Gozo Shioda, and many more aikidoka did, who faced death on a daily basis for years, then returned to civilian life to practice aikido in the way we now practice it, i'll regard the mindset of MMA sportsmen as more valuable/brave than that of aikidoka.