I just shot an MMA fight last weekend, but I was lucky and this venue had amazing light. I used a 50d and 35L as I was anticipating crappy light, but ended up being able to shoot at 1/800,f2.8,ISo800.
Pics from this weekend:
In "normal" MMA setups, I have to shoot 1/320-400,f1.8-2,ISO800-1600. I always use AI Servo mode with spot metering and have good luck getting focus majority of the time. For the most part I use auto focus points when the fighters are moving, and single points when they are on the ground. Getting the right timing however, that is the real trick! I have yet to master that.
I have used the Canon 50 f1.4 before and I found it too long for a lot of shots. I also have the Sigma 30mm f1.4 and it does a fairly good job, just have to crop the shots when the fighters are on the opposite side of the cage.
These pics were taken with the Siggy 30mm f1.4:
I would also have to agree with SnapLocally, the 50d is fine, your glass just was not fast enough for the poor light, this is where primes are your friend. It is nice to have the 24-70 range, but having fast glass is much more important to me.
Good luck! I have another fight to photograph this weekend, they are a lot of fun!