Bold added for emphasis. I have some (flimsy, small sample size) anecdotal evidence to support this observation.
A few hours ago I went to my local GameStop to pre-order the PS4. I discussed the E3 conferences with the clerk. The clerk observed that interest in the PS4 was much stronger than in the Xbox One, and that he, too, would be getting PS4 at launch.
About halfway through the preorder process, the clerk received a call from another customer asking about preorder availability for the console. The clerk tells the customer that there are no preorders left for the XBox One, but that GameStop is still taking preorders for PS4 - after this he hangs up. I found this somewhat curious - (my local) GameStop is anticipating much higher demand for PS4, but isn't even taking XBox One orders? When I asked the clerk about that apparent conflict, this was the response I got (accompanied with what I can only characterize as a knowing smile):
"I could get fired for telling you this... when I said we're out of XBox One orders, it's not what it looked like."
I didn't pursue that line of questioning any further, but the more I've thought about this exchange, the more I'm convinced that there are only a couple of reasonable explanations for this.
(1) My local GameStop is getting more PS4 preorders than XBox One preorders, but the ratio of PS4 units to XBox One units requested by the store is even more lopsided, so the Xbox One has sold out first. This doesn't require any assumptions about the motivations of any party, but does nothing to explain the answer I got from the clerk.
(2) GameStop is aggressively trying to push pre-orderers to choose PS4, and is instructing its staff not to indicate that the XBox One is available for pre-order unless the customer speciflcally asks for it.
(3) The clerk is a fanboy and is trying to steer customers towards the PS4 on his own - based on my previous conversations with the clerk, I don't think this is particularly likely.
Again, this is one person's observations based on interaction with one employee of one GameStop location. I wouldn't draw any conclusions based on this, but if GameStop is, in fact, going to try and steer as many undecided customers as possible toward PS4 (and away from XBox One), the above would fit.