Do you know how much backlash and reputation, and most importantly sales Apple would lose if someone found out that they were doing this? Apple is in a stage where they can do nothing wrong. It is silly of you to think they would purposely slow down devices in an update.
This, this, a thousand times this.
Come on folks, things are starting to get ridiculous around here. Apple is secretly putting an A4 in the iPad Mini and lied during the presentations! (No, there was an inaccurate part number during the presentation). Apple are deliberately holding back a retina display that they could have put in the mini! (No, they picked this design so can't fit a big enough battery & a few more major engineering issues, not to mention costs). Apple is secretly slowing down 3rd gen iPad's to encourage upgrades! (No, if there IS a slowdown, and this is hardly a widespread issue right now, it's either a local problem or maybe a bug in 6.0.1).
We are not living in a spy novel, the most logical explanation is usually the more likely one and Apple aren't going to do anything underhanded that they can get sued to hell for!
Quite aside from the sheer stupidity of such a move why on earth would they need to do so? The iPad is still selling ridiculously well, nothing else is coming near it, so why risk destroying your entire corporate reputation for no actual gain?