Sorry to be so synical but I think some of the current descriptions of Ki are a bit much. I think one of the biggest problems in aikido is the misinterpretation of Ki as "the force".
I have not seen or heard of any Ki related phenomenon that can not be explained by biomechanics.
Surely this is why Ki is related to the tanden - the bodies centre of gravity which may be altered by muscular effort or relaxation to accomplish feats attributed to Ki.
Similarly the unbendable arm is achieved by visualising water or ki flowing from the arm but is in reality an alteration of the way you use the muscles of the arm.
Training with Ki-society and similar aikido stylist reveals this - they are well grounded, but sometimes tend to try to muscle through techniques claiming to 'extend' Ki.
Perhaps the problem is that interpretation of ki has been altered by the influence of the omoto kyo religion. The reason I offer that ki training may be irrelevant to effective aikido is that styles which do not even consider ki demonstrate equal effectiveness and their experienced practitioners are able to peform ki feats equally well due to a learned understanding of biomechanics.