"The store may not be open, but the iPhone/iPad platform itself could hardly be more open to tinkerers of all ages."
Considering it as an Apple product in the vein of the iPhone, with all the limitations that go along with that? Yes. But considering it as a computer? No.
$100 is only required if you want to publish your apps on Apple's App Store. If you just want to tinker you can build the apps for yourself and let friends use them also. I don't recall what the limitations are on that. As for the requirement to own a computer to develop for it -- yeah... that's common.
Not true. Apple requires you to sign your application before the iPhone OS will allow it to be installed. Also, the check is on the device itself, not in iTunes.
There's only two ways to get the apps installed: 1) Pay $100. 2) Crack the device.
Considering it as an Apple product in the vein of the iPhone, with all the limitations that go along with that? Yes. But considering it as a computer? No.