Here is my response to a similar question on another board. Higher Glycemic, the better.
There are a few key reasons. The main ones are that the carbs post-workout should be absorbed quickly to get to the muscles faster. this will cause a greater insulin response which is an anabolic hormone when manipulated properly, and after lifting it is mainly anabolic to muscle and not to fat since the muscles are more sensitive to carbohydrates after a workout due to the upregulation of various enzymes (i believe glut-4). On top of that, the high carbs will quickly blunt the elevated levels of cortisol (which if too high breaks down muscle tissue) that normally rise from lifting, this will allow less muscle loss and more anabolism. Also, potentially, the high carbs can cause greater muscle to be built than w/ out the carbs. The carbs also help transport amino acids in the protein to muscles faster and more efficiently and more of the protein is used towards muscle tissue since there are carbs that are meeting fuel needs rather then protein broken down via gluconeogenesis to meet some of the bodies energy needs.
BTW, the most recent research shows 1-1.5g per kg of bodyweight w/ about 1/3 as much protein is ideal for these benefits. Preworkout protein (about 15 min before lifting), Then post-workout carbs followed w/ a slower protein can probably cause the same benefit as high gi carbs w/ whey isolate or hydrolized protein. (Bcaa's can help as well)