Supporting Edwards views, the worst injury I have caused is accidently throwing someone into a kneeling person on the mat. They were struck in the kidneys and went unconcious immediately (I think they may have stopped breathing as well as they vomitted when they came round - can't remember if there was resucitation, I think there was).
Also, I have dislocated someones shoulder due to the ukemi (they were quite experienced but it was quite a high throw and a bit awkward). Most injuries I have caused to myself (poor ukemi, stabbing myself with live blades, or not moving correctly during an attack).
I think injuries are part and parcel of everyday life. I've had as many injuuries out of the dojo as in it. You should only be afraid of injuring people if you apply technques without control. As Colleen says, with beginners you can't expect a beautiful ukemi, so you have to the technique appropriately. To me there should be a balance between realism and safety (after all its no point putting yourself out of action). Ideally no injuries are caused but if there is too much caution the aikido will suffer.