I think the whole point is that it some people have physical limitations that prevent them from taking what is typically considers "proper ukemi".
The question is..."can they still effectively learn aikido?"
i think they can. We all have strengths and weaknesses. Aikido can be a wonderful tool to discover them and learn how to deal with them more effectively.
It is wrong to ignore our weaknesses or make excuses for not realizing our fullest potential. (Look at Christopher Reeves...what a great role model for anyone of any phyical or mental capabilitites!) From the last interview I saw...He was really saying the same things that O'Sensei was professing! I think he "gets" it and is truely on the path to enlightment!