As always I begin with the caveat that a good story can be read from many different directions. I titled my telling "The Cauldron of Inspiration and the Birth of Taliesin" instead of "Cerridwen and Taliesin" because I feel that she is in service to the real magic in the story--- the muse.
The uncanny witch is an instigator. She gets things going in ways that we can't predict or control. In the form of Cerridwen she seeds the imagination by opening a creative doorway that Gwion, who literally becomes a seed and then seeds her, steps through. I love that Cerridwen lives on an island in the middle of a large lake (Lake Bala is four miles long and one mile wide and sacred in the Welsh tradition). As a reflective body, a lake suggests the play between the surface and the depths and often symbolizes a portal into the unconscious.
Cerridwen brews the potion, but does she get what she wants? The fire flares and the cauldron sputters and suddenly the gifts that she intended for her son Morvran are given instead to Gwion, who is reborn as the great bard Taliesin. In the end she is mother to a wise poet, despite her hostility to Gwion. Something works out perfectly and a marvelous boon is given to the world, but not in the form that Cerridwen intended. She can conjure and bring forth but she cannot control who will be gifted. Such is the muse. Which leads me to Taliesin's riddle and its possible significance. Apparently no one in the court had sufficient knowledge of the wind to come up with that answer, and here the story repeats that central message about the unpredictable power of creative inspiration.
What is the wind? From the Greek pneuma, Hebrew ruach, and Arabic ruh, this wind is the divine breath of creation, the Holy Spirit, the disperser of seeds. It is that unpredictable factor that moves or compels us to change course, perhaps pushing us in the direction of our destiny. It is another metaphor for the inspiration that takes form as the bard Taliesin.
This is a beautifully layered story of echoing themes. I'll end with one more to ponder: the shapeshifting during Cerridwen's pursuit of Gwion. Shapeshifting reveals the protean nature of the material world. One thing can become another because everything is One thing. And what wisdom did the potion impart, to fuel the creativity of the poet? The ability to know past, present, and future and to apprehend the underlying unity of all things. So is this "just" an old fairy tale with a fun bit of magic or a Celtic commentary on the Tao?