I think it was Eddy Cue who said specifically in an interview that they will not do this.
Edit: from the Guardian
He also denies a recent report that Apple is planning to aggressively promote its new streaming service to people buying albums on the iTunes Store. “That’s the wrong way to look at it. You shouldn’t take a customer who’s buying an album, who’s happy buying an album, and try to tell them that what they’re doing is wrong,” he says.
“But we don’t have to, and we shouldn’t try to kill the iTunes Store or kill people that are buying music. If you’re happy buying a few albums a year and that’s the way you’d like to continue doing it … but if we can help you discover some new artists or some new albums through Connect or through listening to Beats 1 radio, great.”
I think they'll get a pretty large amount of trial users simply by advertising it in iTunes - oh you're about to buy this album? Why don't you listen to it and the majority of our catalog free for three months instead?
Other than Android they have a lot of music use cases supported, free of charge - why would you buy?
I honestly don't know how much this will affect sales but Apple are certainly in a position to push users away from those sales and toward the free service.