It essentially has the same problem as a jailbroken iPad/iPhone/Hackintosh. There are costs with supporting issues that arise from this if it's official.
"In my idea, people would be running non-official Apple software without Apple support."
Well then why complain when in all likelihood a 3rd party will give you what you want for Apple products - ala unsupported jailbreak?
"You're dead wrong. If the iPad is to become as pervasive as they would like it to, Apple is going to have to open it up enough so that it can operate as a general purpose computer for the small segment of the populace that wants it."
Given the history with iPod, iTunes, & iPhone I'm going to disagree. All of them are closed systems with the same critics. Yet all of these products have been wildly successful despite that. I'm not saying that this would work with any company, but it works with Apple; it's the part of their company DNA that has proven time and again to work. As I've said in previous posts, there was a time when Apple was more 'open'; and it was a total failure that almost took down the entire company.
"There is no technical reason why Apple can't have their "walled garden" and still let a few people do dangerous things if they want."
I agree but there are a myriad of other reasons as to why they shouldn't such as extra costs, making their content partners (movies, music, and books) happy (I suspect this is a really big reason), and so on. It's not perfect for everyone; but it's worked for non-techies, Apple stock holders, and Apple's partners.
"Actually, they are already doing some of this. It's called the "iPhone SDK.""
Then why complain? Just pay the $99 and be happy.
I suggest moving on and helping either the Chrome OS or Linux hardware movement if you really want officially blessed freedom. You're not going to get that from a mainstream console maker; we are a niche audience.