I don't get why Apple would do this (nor HOW they would do it). All slowing down someone's iPhone would do is make that person get frustrated and want to get a different phone -- and possibly not an iPhone. If, on the other hand, the person simply had a much shorter battery life, at least the iPhone would still be performing properly and perhaps encourage the person to either get it repaired or upgraded.
I would guess it's a tradeoff and Apple feels more users would notice decreased battery life than a slower phone. Having phone that only lasts say 4 hours is immediately noticeable and frustrating, vs slower loading pages, etc. which may be blamed on the network rather than the phone.