Aww man the "Aikido fighting effectiveness" part this thread is so sad. I truly feel for some of you folks. Especially the ones who are giving advice as to why Aikido "can't" work in certain situations after having tried so hard cross training etc. to get things to work. Of course for those who think it can work, don't believe the naysayers for a second, it isn't as hard as most will have you think, but the key is in training smart, not hard or long.
Regarding the OP I think it is good that MMA is here and gives some people an option towards training, fitness and competition if they so desire. It may be the closest thing to "real" empty handed fighting so far and it gives those who need it a creative outlet for releasing pent up energy. I don't however see MMA as being any sort of challenge to Aikido's popularity. People will be attracted to what they like at the time. At some point in life I may feel like buying a Ferrari at another stage I'd want a Rolls Royce. One cannot be said to be better than the other since they address different needs, tastes and desires. In their fundamental role however they are the same, they get you from one point to the next.
A good thing about MMA also is that it has a lot of folks in TMA questioning their training paradigms especially in the area of "fighting" ability. The result has been many TMA folks cross training in MMA and similar methods or flat out leaving TMA and taking up MMA.
The interesting thing that I have found, is that those TMA-ists who cross train but use their experiences to go deeper and train smarter in their traditional arts tend to get breakthroughs and achieve a level of skill in their traditional methods that will never be enjoyed by those who either find the challenge too difficult to get better at their TMA to match the MMA skillsets or leave TMA to join MMA totally to "find what is missing" in their training.
It is no doubt that studying MMA will get one some great practice in some very effective skillsets but I often wonder why people who have done MMA and competed for extensive periods in Judo etc. come to my Aikido class still looking for "that which is missing".
I think MMA and Aikido coexist quite nicely.
Gambatte (truly, some of us need it).