Here's my two cents on this subject:
As I understand it, Tai Sabaki is proper body movement - simply put. If your body posture & dynamics are not congruent with the principle of Aiki, then you will not be able to respond effectively to your opponent with Aikido. Weapons practice is a very effective way to train your body in Tai Sabaki (Suburi for instance).
If you think of this broadly, every physical endeavour has it's own 'Tai Sabaki' - whether it's hitting a baseball, Olympic powerlifting, or dancing - and they also have certain traits in common(Relaxation, centering (balance), proper breathing (Kokyu-ho), timing (Maai), etc).
I also think that Kokyu is an integral aspect of Tai Sabaki, as no movement within Aikido is complete without it. To paraphrase Patrick Cassidy Sensei,'first you must have Tai Sabaki (w/Kokyu), then Ki No Musubi, only with both will you be able to perform with Aiki', or essentially, Tai Sabaki(w/Kokyu) + Ki No Musubi = Aikido, in that order. This was very enlightening for me and I thought I'd share it with everyone else here.
I tend to think of Aikido as the Algebra of the Martial arts: though specifics change (with different systems), the proper form (principles), etc. do not. Meditate on that awhile...
I hope this helps in some way!
~Ward F. Ward