Apple IS going to allow 3rd party apps
To me it seems very very obvious Apple is going to allow 3rd party apps. The release of these guidelines for web apps is just another step in that direction. However, they are going to want to have a firm grip on what is being released, so phone functionality, security and overall usability will not be jeopardized by bad, malicious, unexperienced or lazy developers. Therefore, I am convinced they will, in time, release an iTunes only install tool, with applications for sale (or for free in some cases) through iTunes. They are going to want to sign each and every app that becomes available for compliance with iPhone usability guidelines and security guidelines. AND they are going to want a piece of the pie of software development, taking in a profit for each app being sold.
And this is probably a very smart move. For phones, more so than for computers as Jobs said, interface usability and phone security is trickier than for computers.
Usability
Computers have a fairly well know interface architecture (which is actually not that great, but at least it's known), so app interfaces developed for computers automatically fit in reasonably well with what people expect and have come to know. Not so for the iPhone interface, which is completely new. So extra thought has to go into making applications usable, as Apple has meticulously done for the applications already available. Even experienced software developers (read: the biggest one
) have perpetually shown to be really bad at designing usable interfaces. So they are going to want a whip to beat developers with to focus on usability.
Security and functionality
Imagine people secretly installing backdoors in apps allowing them to e-mail off the contact lists of iPhone owners to spammers. Imagine a sprawling of unusable apps developed by 16 year old pimple faces, crashing iPhones all over the place. This would REALLY be bad for the iPhone.
So don't count on an SDK for the general public. IMO it is all going to be tightly controlled by apple, and this is going to bring really great apps. We just need a little patience. And for those who do not have patience, they can simply stick with 1.0.2