I guess common injuries are knees, shoulders as beginners, toes (depending on the mat and the weight of fellow aikidoka who sometimes might step on you...!
I think injuries also vary by the type of aikido a dojo does. Some work very hard, street- like, some are very soft and flexable and so on. I guess if a mat is very full and there are alot of techniques with movements and rolling there is a bigger chance to get hurt, but that can be reduced by working in groups instead of one on one.
It also depends on how people feel that day... If someone is angry, he might bring that on the mat and it's a cause for disaster. Most senseis warn against that and ask to leave personal life off the mat (as much as possible). But a person for himself- I think that when I train well- that is, move , roll, get up quickly, I am less prone to being hurt because the way my body responds is more with awareness. Instead of someone who takes his time to get up, hardly moves...
Sorry I might not have been too clear on what I meant.
Still, I think there are much less injuries in Aikido than most MA's or sports of other sorts.
Train and enjoy!