Abrac
Oops, never mind.
Yelell, I seem to remember reading somewhere that a bird, moving upward especially, can symbolize Fire. I think it might have been in Robert Place's The Tarot: History, Symbolism, and Divination but I could be way off. The dove on the Ace is connected to the solar wafer and cross, so there could theoretically be a connection.
earthair, it's an interesting idea. Do you have any references from Waite that might indicate, or even hint at, a relationship between that symbol and the Ace?
Could the reversed M be of Waite's own suggestion at sexual symbolism? If the cup is upright, putting an upside down letter on it to read upright wouldn't make sense. When I look at the Thoth deck, a beam of light enters the cup from above, as if impregnating the female element. The Yod, as it were, descends to unite with Mem.
Waite's dove also descends into the cup rather than rising out of it, so could the reversed M signify the female on her back with her "legs" up and open, ready to receive the sacred power from above?