The probable reason the iOS devices are locked down is because that's where Apple's money is now.
If Apple encouraged jail-breaking that would dramatically alter the nature of their product line-up. If the jail-broken phones developed a reputation for crashing, for apps not working properly, or for being a vehicle for rampant piracy, that would taint their brand.
It's a tough position they're put in. As an enthusiast I'd really like to see the iOS hardware sold without locks, specifically for hacking, but Apple isn't interested in that market.
The good news is that whatever Apple pushes hard, be it iPads or iPhones or iPods, puts enormous pressure on the supply channel to deliver parts at scale. Where touch-screen panels used to be stupidly expensive, you can buy them for under $50 from many vendors.
Like an ice-breaker, Apple's enormous volume causes the price of components to plunge across the board. Everything they touch becomes commoditized in time. This is good for consumers, they get better products from third parties, and amazing for hardware hackers that want to build or customize their own gear. Want a quad-core ARM CPU? It's a couple of bucks. Think about that.
You can get an OEM tablet from some random vendor in China that's as good as an iPad 2 hardware-wise, and you can get them cheap. For a little extra, you can get them customized. This is not a bad thing.
Apple will keep their hardware locked down, it's their call, but it opens up enormous opportunities in the open-hardware arena.
