Chris,
It's generally accepted (or claimed, anyway) that Yeats was writing about Michelangelo's version when he did this poem. There's apparently also an Hellenistic Relief that scholars feel influenced the poem. In any event, the subject itself has been so often depicted in classical painting and sculpture, as well as by poets such as Ovid et al, that certainly Yeats was working from a foundation here.
Here's a quote from the link at the bottom of this page. The link's well worth checking out.
According to Charles Madge* the above mentioned Hellenistic relief would have inspired Yeats. Which is evidenced by the flapping of the wings, the emphasis on the web on Leda’s thighs, but foremost by the way in which the swan catches Leda’s neck and presses her face against its breast.
But equally right are all those who traditionally maintain that the poem is inspired by Michelangelo’s Leda. To begin with, Yeats’ Leda does not stand upright, as she does on the Hellenistic relief. She is lying supine, as with Michelangelo. And even when the webs on Leda’s thighs may also appear on the relief, marble has no colour, and it is precisely the resonance of that colour black that is more than echoed in that splendid ’her thighs caressed by the dark webs’. But foremost those ‘terrified fingers’ betray that also the painting of Michelangelo lies at the roots of Yeats’ sonnet. Even when they ward off, rather than wriggle out of pleasure. The mere fact that Yeats’ Leda uses frail fingers rather than full arms to ward off the brutal swan, at once reminds us of the fact that also on the Hellenistic relief Leda does not ward off. With her full arm she rather eagerly extends a helping hand – her wriggling fingers being hidden from view through the thighs.
http://d-sites.net/english/yeats.htm
(robt)