The Surface Pro is growing very well, though I don't think it would be accurate to say that it is signicantly taking away share from the iPad. As far as I know, the SP3 sold ~800k units last quarter, as compared to the iPad's >12 million.

It's really hard to say how Android tablets are doing because the market for them is so diverse.

Overall tablet sales are still going up with low-end tablets (mostly Android) eating up market share.

I think this is probably the key. The rise of $200 90%-as-good-as-iPad tablets is capturing a bunch of the market. And really, if you're not already on iOS, they're >90% as good.