As you know, the biggest problem that is still shopped to the general public is that the deterioration of our muscles and bone density due to zero G is the greatest hindrance to any hope of extended space travel.
As of this moment NASA is still trying to find a substitute for artificial gravity. I know astronauts frequently exercise in zero G environments but they do not offer the same benefits of real gravity.
Here is a proposal I was able to locate at my schools online research database. Warning though, it is Colin Powell worthy (24 pages).
http://www.4frontierscorp.com/dev/assets/00 - MAAE 4906 - Biomechanics Final Project.pdf