Though it's not available anymore, I'd be curious to know how valid it is when developing applications for nowadays PDAs.
Maybe one of the reason the iPhone will be closed to third parties it's because Apple prefer to have enough experience in developing their own software, before writing an application design guide and open the handheld to external developers.
I think the environment in which the iPhone will evolve is so different than Newton's.
Now, everything is connected, worms, malware and phishing also spread on mobile devices, on IM. PDA are now expected to be multimedia player while connectivity comes into multiple wireless flavours, all eager to drain the battery's life (it was an excellent idea to put 2 batteries in the iPhone)
Also cautious users want snappy interface and the shortest time to completion for whatever they want to do because each second spent messing with the UI is another second of exposure to thieves in public transports.
That's a challenge.
I think it's a wise decision for the iPhone to starts its life closed to third parties software.