Re: Aikido & back surgery
I agree with the above adivice. sit down with your doctor and talk about it. I an no one here i guess really knows the extent of the surgery or if you still feel discomfort or pain now.
I have found over th years that becasue Martial arts deal with moving your own body around they can be excellent for injury recovery, if done correctly. so even if your docotr says it is ok when you join a dojo let the teacher know about your surgery. if they are a good teacher then you will be able to work around any issues that arise
as for the breakfall disscussion above, I can be sure but i would expect that many dojos would ask beginners to do heavy breakfalls from day one so it won't be an issue for a while if ever
all the best
hope it all works out for you